scripts/setlocalversion: Improve -dirty check with git-status --no-optional-locks

git-diff-index does not refresh the index for you, so using it for a
"-dirty" check can give misleading results. Commit 6147b1cf19
("scripts/setlocalversion: git: Make -dirty check more robust") tried to
fix this by switching to git-status, but it overlooked the fact that
git-status also writes to the .git directory of the source tree, which
is definitely not kosher for an out-of-tree (O=) build. That is getting
reverted.

Fortunately, git-status now supports avoiding writing to the index via
the --no-optional-locks flag, as of git 2.14. It still calculates an
up-to-date index, but it avoids writing it out to the .git directory.

So, let's retry the solution from commit 6147b1cf19 using this new
flag first, and if it fails, we assume this is an older version of git
and just use the old git-diff-index method.

It's hairy to get the 'grep -vq' (inverted matching) correct by stashing
the output of git-status (you have to be careful about the difference
betwen "empty stdin" and "blank line on stdin"), so just pipe the output
directly to grep and use a regex that's good enough for both the
git-status and git-diff-index version.

Cc: Christian Kujau <lists@nerdbynature.de>
Cc: Guenter Roeck <linux@roeck-us.net>
Suggested-by: Alexander Kapshuk <alexander.kapshuk@gmail.com>
Signed-off-by: Brian Norris <briannorris@chromium.org>
Tested-by: Genki Sky <sky@genki.is>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

scripts/setlocalversion

@@ -73,8 +73,16 @@ scm_version()
 			printf -- '-svn%s' "`git svn find-rev $head`"
 		fi
 
-		# Check for uncommitted changes
-		if git diff-index --name-only HEAD | grep -qv "^scripts/package"; then
+		# Check for uncommitted changes.
+		# First, with git-status, but --no-optional-locks is only
+		# supported in git >= 2.14, so fall back to git-diff-index if
+		# it fails. Note that git-diff-index does not refresh the
+		# index, so it may give misleading results. See
+		# git-update-index(1), git-diff-index(1), and git-status(1).
+		if {
+			git --no-optional-locks status -uno --porcelain 2>/dev/null ||
+			git diff-index --name-only HEAD
+		} | grep -qvE '^(.. )?scripts/package'; then
 			printf '%s' -dirty
 		fi