git-gui: handle non-standard worktree locations

Don't rely on the git worktree being the updir of the gitdir, since it
might not be. Instead, define (and use) a new _gitworktree global
variable, setting it to $GIT_WORK_TREE if present, falling back to
core.worktree if defined, and finally to whatever we guess the correct
worktree is. Getting core.worktree requires the config from the alleged
git dir _gitdir to be loaded early.

Supporting non-standard worktree locations also breaks the git-gui
assumption (made when calling gitk) that the worktree was the dirname of
$_gitdir and that, by consequence, the git dir could be set to the tail
of $_gitdir once we changed to the worktree root directory. Therefore,
we need to export a GIT_DIR environment variable set to the full,
normalized path of $_gitdir instead. We also skip changing to the worktree
directory if it's empty (i.e. if we're working on a bare repository).

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

git-gui.sh

@@ -121,6 +121,7 @@ unset oguimsg
 
 set _appname {Git Gui}
 set _gitdir {}
+set _gitworktree {}
 set _gitexec {}
 set _githtmldir {}
 set _reponame {}
@@ -1100,25 +1101,41 @@ if {![file isdirectory $_gitdir]} {
 	error_popup [strcat [mc "Git directory not found:"] "\n\n$_gitdir"]
 	exit 1
 }
+# _gitdir exists, so try loading the config
+load_config 0
+apply_config
+# try to set work tree from environment, falling back to core.worktree
+if {[catch { set _gitworktree $env(GIT_WORK_TREE) }]} {
+	set _gitworktree [get_config core.worktree]
+}
 if {$_prefix ne {}} {
-	regsub -all {[^/]+/} $_prefix ../ cdup
+	if {$_gitworktree eq {}} {
+		regsub -all {[^/]+/} $_prefix ../ cdup
+	} else {
+		set cdup $_gitworktree
+	}
 	if {[catch {cd $cdup} err]} {
 		catch {wm withdraw .}
 		error_popup [strcat [mc "Cannot move to top of working directory:"] "\n\n$err"]
 		exit 1
 	}
+	set _gitworktree [pwd]
 	unset cdup
 } elseif {![is_enabled bare]} {
 	if {[lindex [file split $_gitdir] end] ne {.git}} {
 		catch {wm withdraw .}
 		error_popup [strcat [mc "Cannot use funny .git directory:"] "\n\n$_gitdir"]
 		exit 1
 	}
-	if {[catch {cd [file dirname $_gitdir]} err]} {
+	if {$_gitworktree eq {}} {
+		set _gitworktree [file dirname $_gitdir]
+	}
+	if {[catch {cd $_gitworktree} err]} {
 		catch {wm withdraw .}
-		error_popup [strcat [mc "No working directory"] " [file dirname $_gitdir]:\n\n$err"]
+		error_popup [strcat [mc "No working directory"] " $_gitworktree:\n\n$err"]
 		exit 1
 	}
+	set _gitworktree [pwd]
 }
 set _reponame [file split [file normalize $_gitdir]]
 if {[lindex $_reponame end] eq {.git}} {
@@ -1935,6 +1952,7 @@ set starting_gitk_msg [mc "Starting gitk... please wait..."]
 
 proc do_gitk {revs {is_submodule false}} {
 	global current_diff_path file_states current_diff_side ui_index
+	global _gitworktree
 
 	# -- Always start gitk through whatever we were loaded with.  This
 	#    lets us bypass using shell process on Windows systems.
@@ -1955,8 +1973,10 @@ proc do_gitk {revs {is_submodule false}} {
 		set pwd [pwd]
 
 		if {!$is_submodule} {
-			cd [file dirname [gitdir]]
-			set env(GIT_DIR) [file tail [gitdir]]
+			if {$_gitworktree ne {}} {
+				cd $_gitworktree
+			}
+			set env(GIT_DIR) [file normalize [gitdir]]
 		} else {
 			cd $current_diff_path
 			if {$revs eq {--}} {
@@ -2032,6 +2052,7 @@ proc do_git_gui {} {
 }
 
 proc do_explore {} {
+	global _gitworktree
 	set explorer {}
 	if {[is_Cygwin] || [is_Windows]} {
 		set explorer "explorer.exe"
@@ -2041,7 +2062,7 @@ proc do_explore {} {
 		# freedesktop.org-conforming system is our best shot
 		set explorer "xdg-open"
 	}
-	eval exec $explorer [list [file nativename [file dirname [gitdir]]]] &
+	eval exec $explorer $_gitworktree &
 }
 
 set is_quitting 0
@@ -2405,8 +2426,6 @@ proc show_less_context {} {
 ##
 ## ui construction
 
-load_config 0
-apply_config
 set ui_comm {}
 
 # -- Menu Bar
@@ -3492,7 +3511,7 @@ unset i
 set file_lists($ui_index) [list]
 set file_lists($ui_workdir) [list]
 
-wm title . "[appname] ([reponame]) [file normalize [file dirname [gitdir]]]"
+wm title . "[appname] ([reponame]) [file normalize $_gitworktree]"
 focus -force $ui_comm
 
 # -- Warn the user about environmental problems.  Cygwin's Tcl