util/release: Improve git worktree checks

The bash script `genrelnotes` checks for `.git` to be present to
determine whether the current directory is the top directory of a git
worktree. This check is rather weak and doesn't handle many edge cases
like that of a broken gitfile.

Add a proper `git rev-parse` call to check the condition.

Change-Id: I32b06ca982d55fd8e88e55651b6bc53014905823
Signed-off-by: Alex Thiessen <alex.thiessen.de+coreboot@gmail.com>
Reviewed-on: https://review.coreboot.org/23252
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

util/release/genrelnotes

@@ -32,7 +32,8 @@ then
 	exit 1
 fi
 
-if [ ! -e ".git" ];then
+if ! { cdup="$(git rev-parse --show-cdup 2>/dev/null)" && [ -z "${cdup}" ]; }
+then
 	echo "ERROR: This is not the top directory of a git repo.  Exiting."
 	exit 1
 fi