git-gui: Handle failure of core.worktree to identify the working directory.

Commit 21985a11 'git-gui: handle non-standard worktree locations' attempts
to use either GIT_WORK_TREE or core.worktree to set the _gitworktree
variable but these may not be set which leads to a failure to launch
gitk to review history. Use _gitdir to set the location for a standard
git layout where the parent of the .git directory is the working tree.
Signed-off-by: NPat Thoyts <patthoyts@users.sourceforge.net>

git-gui.sh

@@ -1158,6 +1158,9 @@ 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 $_gitdir]]
+	}
 }
 if {$_prefix ne {}} {
 	if {$_gitworktree eq {}} {