git-gui: work from the .git dir

When git-gui is run from a .git dir, _gitdir would be set to "." by
rev-parse, something that confuses the worktree detection.

Fix by expanding the value of _gitdir to pwd in this special case.

Signed-off-by: Giuseppe Bilotta <giuseppe.bilotta@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

git-gui.sh

@@ -1074,6 +1074,8 @@ if {[catch {
 		set _prefix {}
 		}]
 	&& [catch {
+		# beware that from the .git dir this sets _gitdir to .
+		# and _prefix to the empty string
 		set _gitdir [git rev-parse --git-dir]
 		set _prefix [git rev-parse --show-prefix]
 	} err]} {
@@ -1082,6 +1084,14 @@ if {[catch {
 	choose_repository::pick
 	set picked 1
 }
+
+# we expand the _gitdir when it's just a single dot (i.e. when we're being
+# run from the .git dir itself) lest the routines to find the worktree
+# get confused
+if {$_gitdir eq "."} {
+	set _gitdir [pwd]
+}
+
 if {![file isdirectory $_gitdir] && [is_Cygwin]} {
 	catch {set _gitdir [exec cygpath --windows $_gitdir]}
 }