Previously, when run in a subdirectory, gitk would show the name
of this subdirectory as title, which was misleading. When run with
GIT_DIR set, it would show the cwd, which is even more misleading.

In case of non-bare repos, the .git suffix in the path is skipped.
Signed-off-by: NZbigniew Jędrzejewski-Szmek <zbyszek@in.waw.pl>