Use shallow fetches when running cherry-picks in CI

build-support/bin/cherry_pick.sh

@@ -13,6 +13,22 @@ function fail {
   exit "${2-1}"
 }
 
+function git_fetch_with_depth {
+    DEPTH="$1"
+    REF="$2"
+
+    # setting --depth will forcibly truncate history, which is fine in the temporary checkout on CI,
+    # but would be annoying locally: people will have crafted the check-out to their liking
+    # (typically the full history), and this script is running interactively anyway
+    if [[ $CI = 1 ]]; then
+        DEPTH_ARGS=("--depth=$DEPTH")
+    else
+        DEPTH_ARGS=()
+    fi
+
+    git fetch "${DEPTH_ARGS[@]}" https://github.com/pantsbuild/pants "$REF"
+}
+
 if [[ -z $(which gh 2> /dev/null) ]]; then
   fail "Requires the GitHub CLI: https://cli.github.com/"
 fi
@@ -51,7 +67,8 @@ COMMIT=$(gh pr view "$PR_NUM" --json mergeCommit --jq '.mergeCommit.oid')
 if [[ -z $COMMIT ]]; then
   fail "Wasn't able to retrieve merge commit for $PR_NUM."
 fi
-git fetch https://github.com/pantsbuild/pants "$COMMIT"
+# Both the commit and its parent are required, to compute the diff to apply when cherry picking
+git_fetch_with_depth 2 "$COMMIT"
 
 TITLE=$(gh pr view "$PR_NUM" --json title --jq '.title')
 CATEGORY_LABEL=$(gh pr view "$PR_NUM" --json labels --jq '.labels.[] | select(.name|test("category:.")).name')
@@ -68,7 +85,9 @@ BODY_FILE=$(mktemp "/tmp/github.cherrypick.$PR_NUM.XXXXXX")
 gh pr view "$PR_NUM" --json body --jq '.body' > "$BODY_FILE"
 
 for MILESTONE in $MILESTONES; do
-  git fetch https://github.com/pantsbuild/pants "$MILESTONE" || continue
+  # Only the HEAD is required to be able to create the cherry-pick branch, so there's no need to
+  # spend time cloning the whole history
+  git_fetch_with_depth 1 "$MILESTONE" || continue
 
   PR_CREATE_CMD=(gh pr create --base "$MILESTONE" --title "$TITLE (Cherry-pick of #$PR_NUM)" --label "$CATEGORY_LABEL" --body-file "$BODY_FILE")
   while IFS= read -r REVIEWER; do PR_CREATE_CMD+=(--reviewer "$REVIEWER"); done <<< "$REVIEWERS"