gitk: Fix "git gui blame" invocation when called from top-level directory

gitk: Fix "git gui blame" invocation when called from top-level directory

When run in the top-level directory of a git repository, "git
rev-parse --git-dir" doesn't return an absolute path, but merely
".git", so the selected file for "git gui blame" has a relative path.
The function make_relative then tries to make the already relative
path relative, which results in a path like "../../../../Makefile"
with as many ".." as there are elements of [pwd].

This regression was introduced by commit 9712b81 (gitk: Fix bugs in
blaming code, 2008-12-06), which fixed "git gui blame" when called from
subdirs.

This also fixes it for bare repositories.

Signed-off-by: Markus Heidelberg <markus.heidelberg@web.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>