gitk: Simplify calculation of gitdir

gitk: Simplify calculation of gitdir

Since 5024baa ([PATCH] Make gitk work when launched in a subdirectory,
2007-01-09), gitk has used 'git rev-parse --git-dir' to find the .git
directory. However, gitk still first checks for the $GIT_DIR
environment variable and that the value returned from git-rev-parse
does not point to a file. Since git-rev-parse does both of these
checks already, the checks can safely be removed from gitk. This makes
the gitdir procedure small enough to inline.

This cleanup introduces a UI regression in that the error message will
now be "Cannot find a git repository here." even in the case where
GIT_DIR points to a file, for which the error message was previously
"Cannot find the git directory \"%s\".". It should be noted, though,
that even before this patch, 'gitk --git-dir=path/to/some/file' would
give the former error message.

Signed-off-by: Martin von Zweigbergk <martin.von.zweigbergk@gmail.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>