refactor(gitcommit): use path_join for file paths

Add GitUtils.path_join to handle platform-specific separators and
normalization. Replace manual string concatenation in git.lua and
tools/git.lua with the new helper.

Author: jinzhongjia

lua/codecompanion/_extensions/gitcommit/git.lua

@@ -38,8 +38,7 @@ function Git.is_repository()
   end
 
   local function check_git_dir(path)
-    local sep = package.config:sub(1, 1)
-    local git_path = path .. sep .. ".git"
+    local git_path = GitUtils.path_join(path, ".git")
     local stat = vim.uv.fs_stat(git_path)
     return stat ~= nil
   end
@@ -77,8 +76,7 @@ function Git.is_amending()
       return false
     end
 
-    local path_sep = package.config:sub(1, 1)
-    local commit_editmsg = git_dir .. path_sep .. "COMMIT_EDITMSG"
+    local commit_editmsg = GitUtils.path_join(git_dir, "COMMIT_EDITMSG")
     local stat = vim.uv.fs_stat(commit_editmsg)
     if not stat then
       return false

lua/codecompanion/_extensions/gitcommit/git_utils.lua

@@ -220,6 +220,38 @@ function M.is_windows()
   return vim.uv.os_uname().sysname == "Windows_NT"
 end
 
+---Join path parts with platform-specific separator
+---Normalizes all parts to forward slashes internally, then converts to platform format
+---@param ... string Path parts to join
+---@return string joined_path
+function M.path_join(...)
+  local parts = { ... }
+  if #parts == 0 then
+    return ""
+  end
+
+  -- Normalize all parts to use forward slashes
+  local normalized = {}
+  for i, part in ipairs(parts) do
+    normalized[i] = part:gsub("\\+", "/")
+  end
+
+  -- Remove leading/trailing slashes from middle parts
+  for i = 2, #normalized - 1 do
+    normalized[i] = normalized[i]:gsub("^/", ""):gsub("/$", "")
+  end
+
+  -- Join with forward slashes
+  local result = table.concat(normalized, "/")
+
+  -- Convert to Windows backslashes if on Windows
+  if M.is_windows() then
+    result = result:gsub("/", "\\")
+  end
+
+  return result
+end
+
 ---Quote a string for shell command (cross-platform)
 ---@param str string The string to quote
 ---@param force_windows? boolean Force Windows quoting style (for testing)

lua/codecompanion/_extensions/gitcommit/tools/git.lua

@@ -36,8 +36,7 @@ local function get_gitignore_path()
   if not git_dir or git_dir == "" then
     return nil
   end
-  local sep = package.config:sub(1, 1)
-  return git_dir .. sep .. ".gitignore"
+  return GitUtils.path_join(git_dir, ".gitignore")
 end
 
 function GitTool.get_gitignore()

tests/test_git_utils.lua

@@ -592,6 +592,104 @@ T["shell_quote_windows"]["handles spaces"] = function()
   h.eq('"hello world"', result)
 end
 
+T["path_join"] = new_set()
+
+T["path_join"]["joins two path parts"] = function()
+  local result = child.lua([[
+    local GitUtils = require("codecompanion._extensions.gitcommit.git_utils")
+    local result = GitUtils.path_join("src", "main.lua")
+    local has_sep = GitUtils.is_windows() and result:find("\\") ~= nil or result:find("/") ~= nil
+    return has_sep and result:match("main%.lua") ~= nil
+  ]])
+  h.eq(true, result)
+end
+
+T["path_join"]["joins multiple path parts"] = function()
+  local result = child.lua([[
+    local GitUtils = require("codecompanion._extensions.gitcommit.git_utils")
+    local result = GitUtils.path_join("src", "components", "ui", "Button.tsx")
+    return result:match("src") ~= nil
+        and result:match("components") ~= nil
+        and result:match("ui") ~= nil
+        and result:match("Button%.tsx") ~= nil
+  ]])
+  h.eq(true, result)
+end
+
+T["path_join"]["handles windows backslashes"] = function()
+  local result = child.lua([[
+    local GitUtils = require("codecompanion._extensions.gitcommit.git_utils")
+    local result = GitUtils.path_join("C:\\\\Users", "test", "file.lua")
+    local has_backslash = result:find("\\") ~= nil
+    local has_users = result:find("Users") ~= nil
+    local has_test = result:find("test") ~= nil
+    local has_file = result:find("file%.lua") ~= nil
+    return has_backslash and has_users and has_test and has_file
+  ]])
+  h.eq(true, result)
+end
+
+T["path_join"]["handles mixed separators"] = function()
+  local result = child.lua([[
+    local GitUtils = require("codecompanion._extensions.gitcommit.git_utils")
+    local result = GitUtils.path_join("C:/project", "src", "main.lua")
+    return result:match("src") ~= nil and result:match("main%.lua") ~= nil
+  ]])
+  h.eq(true, result)
+end
+
+T["path_join"]["handles trailing slashes"] = function()
+  local result = child.lua([[
+    local GitUtils = require("codecompanion._extensions.gitcommit.git_utils")
+    local result = GitUtils.path_join("src/", "test/")
+    return not result:match("//") and result:match("src") ~= nil and result:match("test") ~= nil
+  ]])
+  h.eq(true, result)
+end
+
+T["path_join"]["handles leading slashes in middle parts"] = function()
+  local result = child.lua([[
+    local GitUtils = require("codecompanion._extensions.gitcommit.git_utils")
+    local result = GitUtils.path_join("src", "/utils.lua")
+    return not result:match("/utils") and result:match("utils%.lua") ~= nil
+  ]])
+  h.eq(true, result)
+end
+
+T["path_join"]["handles single part"] = function()
+  local result = child.lua([[
+    local GitUtils = require("codecompanion._extensions.gitcommit.git_utils")
+    return GitUtils.path_join("test.lua") == "test.lua"
+  ]])
+  h.eq(true, result)
+end
+
+T["path_join"]["returns empty string for no parts"] = function()
+  local result = child.lua([[
+    local GitUtils = require("codecompanion._extensions.gitcommit.git_utils")
+    return GitUtils.path_join() == ""
+  ]])
+  h.eq(true, result)
+end
+
+T["path_join"]["handles empty middle parts"] = function()
+  local result = child.lua([[
+    local GitUtils = require("codecompanion._extensions.gitcommit.git_utils")
+    local result = GitUtils.path_join("src", "", "test.lua")
+    return not result:match("//") and result:match("test%.lua") ~= nil
+  ]])
+  h.eq(true, result)
+end
+
+T["path_join"]["preserves dots in path"] = function()
+  local result = child.lua([[
+    local GitUtils = require("codecompanion._extensions.gitcommit.git_utils")
+    local result = GitUtils.path_join("..", "utils.lua")
+    return result:match("%.%.") ~= nil and result:match("utils%.lua") ~= nil
+  ]])
+  h.eq(true, result)
+end
+
 T["is_windows"] = new_set()
 
 T["is_windows"]["returns boolean"] = function()