git-gui: Allow [gitdir ...] to act as [file join [gitdir] ...].
git-gui: Allow [gitdir ...] to act as [file join [gitdir] ...].

Because it is such a common idiom to use [gitdir] along with [file join]
to locate the path of an item within the .git directory of the current
repository we might as well allow gitdir to act as a wrapper for the
file join operation.

Signed-off-by: Shawn O. Pearce <f01032651c12a22b2103346ba96b3b6b231f9d3c@spearce.org>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git