util/gitconfig: update pre-commit script

The previous pre-commit script generates an error when a commit doesn't
actually generate a patch file.  This happens in rare instances when the
only change in a patch is to update file permissions.

Update the pre-commit script to verify that there's actually a patch
before calling checkpatch.  Also print that we're running checkpatch
so that it doesn't look like odd failures come from lint-stable.

Fixes Bug #132 - Problem with `lint-stable` when changing mode of file

Change-Id: I142610b6fc984b2b14fd6c634bc4e7f2880ba987
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/20781
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

util/gitconfig/pre-commit

@@ -1,3 +1,9 @@
 #!/bin/sh
 %MAKE% lint-stable
-git diff --cached | util/lint/checkpatch.pl --no-signoff -q -
+
+PATCHDIFF=$(git diff --cached)
+if echo "$PATCHDIFF" | grep -q "@@"; then
+	echo
+	echo "Running checkpatch"
+	echo "$PATCHDIFF" | util/lint/checkpatch.pl --no-signoff -q -
+fi