[fix] Gitk fails to open on a bare Git repository on Windows

Running 'git rev-parse --show-toplevel' fails on a bare git repo.
This causes gitk to fail to startup.

Signed-off-by: Remy Bohmer <[email protected]>

Author: remybohmer

gitk-git/gitk

@@ -12599,7 +12599,9 @@ set cdup {}
 if {[expr {[exec git rev-parse --is-inside-work-tree] == "true"}]} {
     set cdup [exec git rev-parse --show-cdup]
 }
-set worktree [exec git rev-parse --show-toplevel]
+if {$hasworktree} {
+    set worktree [exec git rev-parse --show-toplevel]
+}
 setcoords
 makewindow
 catch {