tools/pre-commit: Don't use stashes

Using stashes in pre-commit hook had the following demerits:
- when stash was not needed, it was not created, so some other stash
  created by user was poped on exit -- this was a source of great
  confusion;
- files with unstaged changes were touched, which caused unwanted
  side effects with some tools like editors or build systems.

This commit changes the way we get the clean source by checking out
index tree to temporary dir without modifying working tree. This
might be a bit slower for large repositories, but in my tests for
this repo it is within 5%.

Change-Id: Ic32fc2f52958b63131fcbe363ea47fca13099ea3

tools/pre-commit

@@ -3,11 +3,30 @@
 # install me this way:
 # cp pre-commit "$(git rev-parse --git-dir)/hooks/"
 
-git stash -q --keep-index
-trap 'git stash pop -q' EXIT
 
-STATUS=0
+gitdir="$(readlink -f $(git rev-parse --git-dir))"
-FILES="$(git diff --cached --name-only --diff-filter=AM | grep -E '\.py$')"
+
+tmpdir=
+cleanup_tmpdir()
+{
+        [ -z "$tmpdir" ] || rm -rf -- "$tmpdir"
+        exit "$@"
+}
+tmpdir=$(mktemp -dt "${0##*/}.XXXXXXXX")
+trap 'cleanup_tmpdir $?' EXIT
+trap 'clenaup_tmpdir 143' HUP INT QUIT PIPE TERM
+
+
+
+git checkout-index -a --prefix="$tmpdir/"
+cd "$tmpdir"
+
+indexed_files()
+{
+    git --git-dir="$gitdir" diff --cached --name-only --diff-filter=AM
+}
+
+FILES="$(indexed_files | grep -E '\.py$')"
 if [ -n "$FILES" ]; then
     pylint $FILES || STATUS=1
     if grep -nEH --color '(import pdb|pdb.set_trace)' $FILES; then
@@ -16,7 +35,7 @@ if [ -n "$FILES" ]; then
     fi
 fi
 
-FILES="$(git diff --cached --name-only --diff-filter=AM | grep -E '\.(py|html|js)$')"
+FILES="$(indexed_files | grep -E '\.(py|html|js)$')"
 if [ -n "$FILES" ]; then
     if grep -nEH --color '\s+$' $FILES; then
         echo "Please remove trailing spaces"