git-gui: Don't switch branches if changing to the current branch.

Its pointless to switch to the current branch, so don't do it. We
are already on it and the current index and working directory should
just be left alone.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

git-gui.sh

@@ -2271,6 +2271,13 @@ The rescan will be automatically started now.
 		return
 	}
 
+	# -- Don't do a pointless switch.
+	#
+	if {$current_branch eq $new_branch} {
+		unlock_index
+		return
+	}
+
 	if {$repo_config(gui.trustmtime) eq {true}} {
 		switch_branch_stage2 {} $new_branch
 	} else {