Improve pre-commit versioning hook

Author: livid

Planet/versioning.xcconfig

@@ -1 +1 @@
-CURRENT_PROJECT_VERSION = 2617
+CURRENT_PROJECT_VERSION = 2619

git-hooks/pre-commit.sh

@@ -1,5 +1,34 @@
 #!/bin/sh
-buildNumber=`git rev-list --count main`
-echo "CURRENT_PROJECT_VERSION = $buildNumber" > Planet/versioning.xcconfig
-echo "Set CURRENT_PROJECT_VERSION to $buildNumber"
-git add Planet/versioning.xcconfig
+
+set -eu
+
+repo_root=$(git rev-parse --show-toplevel)
+git_dir=$(git rev-parse --git-dir)
+case "$git_dir" in
+    /*) ;;
+    *) git_dir="$repo_root/$git_dir" ;;
+esac
+
+actual_index="$git_dir/index"
+current_index="${GIT_INDEX_FILE:-$actual_index}"
+version_file="Planet/versioning.xcconfig"
+build_number=$(( $(git rev-list --count HEAD) + 1 ))
+version_line="CURRENT_PROJECT_VERSION = $build_number"
+
+cd "$repo_root"
+
+current_line=""
+if [ -f "$version_file" ]; then
+    current_line=$(cat "$version_file")
+fi
+
+if [ "$current_line" != "$version_line" ]; then
+    printf '%s\n' "$version_line" > "$version_file"
+    printf 'Set CURRENT_PROJECT_VERSION to %s\n' "$build_number"
+    git add -- "$version_file"
+
+    # Partial commits can run hooks against a temporary index.
+    if [ "$current_index" != "$actual_index" ]; then
+        GIT_INDEX_FILE="$actual_index" git add -- "$version_file"
+    fi
+fi