git-gui: Use PWD if it exists on Mac OS X

The current working directory is set to / when git-gui is invoked
using the Git Gui.app bundle on Mac OS X. This means that if it is
launched from a directory which contains a repository then git-gui
won't automatically find it unless the repository happens to be
located in /.

The PWD environment variable is however preserved if the bundle is
invoked using open(1). If git-gui would check for PWD then a user
could for example type open -a 'Git Gui' on a command line in order to
launch the program and it would automatically find the repository.

Teach git-gui to use the PWD environment variable on Mac OS X.
Signed-off-by: NMarcus Karlsson <mk@acc.umu.se>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>
Signed-off-by: NPat Thoyts <patthoyts@users.sourceforge.net>

macosx/AppMain.tcl

@@ -12,7 +12,9 @@ if {[file tail [lindex $argv 0]] eq {gitk}} {
 } else {
 	set argv0 [file join $gitexecdir [file tail [lindex $argv 0]]]
 	set AppMain_source [file join $gitguilib git-gui.tcl]
-	if {[pwd] eq {/}} {
+	if {[info exists env(PWD)]} {
+		cd $env(PWD)
+	} elseif {[pwd] eq {/}} {
 		cd $env(HOME)
 	}
 }