avoid unnecessary long filter range, if there aren't any relevant new commits

Author: DanielBauten

lib/git-subrepo

@@ -806,6 +806,7 @@ subrepo:branch() {
     if [[ -z "$prev_commit" ]]; then # i.e. there are no new reachable commits
       o "Create branch '$branch' for subrepo commit $subrepo_commit."
       prev_commit="$subrepo_commit"
+      first_gitrepo_commit="$subrepo_commit" # force neutralization of $filter to avoid unnecessary large range (see below)
     else
       o "Create branch '$branch' for this new commit set $prev_commit."
     fi