actions: depth: add action, gets fetch depth

Signed-off-by: Jorge Marques <jorge.marques@analog.com>

Author: gastmaier

ci/actions/depth/README.md

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+Depth
+=====
+
+Sets the FETCH_DEPTH variable, to be passed to GitHub's checkout action.
+
+Usage:
+
+```
+on: workflow_call
+jobs:
+  checks:
+    runs-on: [self-hosted, v1]
+
+    steps:
+    - uses: analogdevicesinc/doctools/range@v1
+    - uses: actions/checkout@v4
+      with:
+        fetch-depth: ${{ env.FETCH_DEPTH }}
+```

ci/actions/depth/action.yml

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+name: Git depth
+description: Get fetch depth from GitHub context
+
+# behaviour:
+# github.event.pull_request.head.sha is always used to detect the PR case.
+# Fetch depth is number of commits + 1 (base commit)
+
+runs:
+  using: composite
+  steps:
+    - name: Get commit depth
+      shell: bash
+      run: |
+        if [[ "${{ github.event.pull_request.commits }}" ]]; then
+          echo "set FETCH_DEPTH based on github.event.pull_request.commits"
+          echo "FETCH_DEPTH=$(( ${{ github.event.pull_request.commits }} + 1 ))" >> $GITHUB_ENV
+        elif [[ ! "${{ github.event.before }}" =~ "0000000000000000" ]] && [[ "${{ github.event.forced }}" == "false" ]]; then
+          echo "set FETCH_DEPTH based on github.event.before"
+          commits_sha='${{ toJSON(github.event.commits.*.id) }}'
+          FETCH_DEPTH=$(( $(jq <<<"$commits_sha" length) + 1 ))
+          echo "FETCH_DEPTH=$FETCH_DEPTH" >> $GITHUB_ENV
+        else
+          echo "setting FETCH_DEPTH as 6 (fallback, either force-push, push-new-branch, or unknown event type)"
+          echo "FETCH_DEPTH=6" >> $GITHUB_ENV
+          echo "FETCH_DEPTH_FALLBACK=true" >> $GITHUB_ENV
+        fi
+