git-gui: Allow gitk to be started on Cygwin with native Tcl/Tk

gitk expects $env(GIT_DIR) to be valid as both a path that core Git
and Tcl/Tk can resolve to a valid directory, but it has no special
handling for Cygwin style UNIX paths and Windows style paths.  So
we need to do that for gitk and ensure that only relative paths are
fed to it, thus allowing both Cygwin style and UNIX style paths to
be resolved.

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

git-gui.sh

@@ -1468,7 +1468,27 @@ proc do_gitk {revs} {
 	if {! [file exists $exe]} {
 		error_popup "Unable to start gitk:\n\n$exe does not exist"
 	} else {
+		global env
+
+		if {[info exists env(GIT_DIR)]} {
+			set old_GIT_DIR $env(GIT_DIR)
+		} else {
+			set old_GIT_DIR {}
+		}
+
+		set pwd [pwd]
+		cd [file dirname [gitdir]]
+		set env(GIT_DIR) [file tail [gitdir]]
+
 		eval exec $cmd $revs &
+
+		if {$old_GIT_DIR eq {}} {
+			unset env(GIT_DIR)
+		} else {
+			set env(GIT_DIR) $old_GIT_DIR
+		}
+		cd $pwd
+
 		ui_status $::starting_gitk_msg
 		after 10000 {
 			ui_ready $starting_gitk_msg