Misc size regression script improvements. (#16268)

* Misc size regression script improvements.

- Sets environment variable SIZE_REGRESSION_EXECUTING during execution
  so hook scripts like `post-checkout` may skip processing.
- Forces checkout of the target branch, including removal of all
  temporary object files in the process.
- Prints out a warning on exit stating that the git repository is in an
  indeterminate state, and the user needs to swap back to whatever
  intended branch they were working with originally.

* Remove `git clean`

util/size_regression.sh

@@ -11,6 +11,8 @@ dest_ref="develop"
 ignore_ref="master"
 unset skip_zero
 
+export SIZE_REGRESSION_EXECUTING=1
+
 function usage() {
     echo "Usage: $(basename "$0") [-h] [-j <jobs>] [-s <source>] [-d <dest>] [-n] planck/rev6:default"
     echo "    -h           : Shows this usage page."
@@ -23,9 +25,24 @@ function usage() {
 }
 
 if [[ ${#} -eq 0 ]]; then
-   usage
+    usage
+    exit 0
 fi
 
+unset cleanup_completed
+_internal_cleanup() {
+    if [[ -z "${cleanup_completed:-}" ]] ; then
+        echo
+        echo
+        echo 'Your git repository is in an indeterminate state!' >&2
+        echo 'Make sure you swap to your intended branch.' >&2
+        echo
+        unset SIZE_REGRESSION_EXECUTING
+    fi
+    cleanup_completed=1
+}
+trap _internal_cleanup EXIT HUP INT
+
 while getopts "hj:s:d:i:n" opt "$@" ; do
     case "$opt" in
         h) usage; exit 0;;
@@ -49,7 +66,8 @@ function build_executor() {
         revision=$(echo $line | cut -d' ' -f1)
 
         make distclean >/dev/null 2>&1
-        git checkout $revision >/dev/null 2>&1 || { echo "Failed to check out revision ${revision}" >&2 ; exit 1 ; }
+
+        git checkout -f $revision >/dev/null 2>&1 || { echo "Failed to check out revision ${revision}" >&2 ; exit 1 ; }
         make -j${job_count} $keyboard_target >/dev/null 2>&1 || true
         file_size=$(arm-none-eabi-size .build/*.elf 2>/dev/null | awk '/elf/ {print $1}' 2>/dev/null || true)