build: Fix ChangeLog building for builds from other worktrees.

* Makefile.am (gen-ChangeLog): Test for existance of ".git" regardless
on whether it is a file or directory.

git worktree create a plain file ".git" and not a ".git" directory.
Thus we can't check for the existance of a directory.

• Signed-off-by: Werner Koch <wk@gnupg.org>