git-gui: correct assignment of work-tree

git-gui currently uses its own logic to determine the work-tree setting
but 'git rev-parse --toplevel' directly returns git's work-tree value
by calling get_git_work_tree() and is therefore always correct.

This fixes an inability to handle some repository configurations. In
particular where .git is a file containing a path to the real directory
(a cross-platform symbolic link).
To continue to support older versions than 1.7.0, setting the work-tree
by normalizing the --show-cdup value is more reliable as git-dir might be
outside the work-tree entirely.
Signed-off-by: NPat Thoyts <patthoyts@users.sourceforge.net>

git-gui.sh

@@ -1194,13 +1194,22 @@ if {![file isdirectory $_gitdir]} {
 # _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 $env(GIT_WORK_TREE) }]} {
-	set _gitworktree [get_config core.worktree]
-	if {$_gitworktree eq ""} {
-		set _gitworktree [file dirname [file normalize $_gitdir]]
+
+# v1.7.0 introduced --show-toplevel to return the canonical work-tree
+if {[package vsatisfies $_git_version 1.7.0]} {
+	set _gitworktree [git rev-parse --show-toplevel]
+} else {
+	# try to set work tree from environment, core.worktree or use
+	# cdup to obtain a relative path to the top of the worktree. If
+	# run from the top, the ./ prefix ensures normalize expands pwd.
+	if {[catch { set _gitworktree $env(GIT_WORK_TREE) }]} {
+		set _gitworktree [get_config core.worktree]
+		if {$_gitworktree eq ""} {
+			set _gitworktree [file normalize ./[git rev-parse --show-cdup]]
+		}
 	}
 }
+
 if {$_prefix ne {}} {
 	if {$_gitworktree eq {}} {
 		regsub -all {[^/]+/} $_prefix ../ cdup