git-gui: Disable pull menu items when the index is locked.

If we have the index locked then no pull command is allowed to proceed
(as it would fail to get the index lock itself).  So disable the pull
menu items when we are doing any index based operations.
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>

git-gui

@@ -932,7 +932,7 @@ proc populate_remote_menu {m pfx op} {
 }
 
 proc populate_pull_menu {m} {
-	global gitdir repo_config all_remotes mainfont
+	global gitdir repo_config all_remotes mainfont disable_on_lock
 
 	foreach remote $all_remotes {
 		set rb {}
@@ -961,6 +961,8 @@ proc populate_pull_menu {m} {
 				-label "Branch $rb_short from $remote..." \
 				-command [list pull_remote $remote $rb] \
 				-font $mainfont
+			lappend disable_on_lock \
+				[list $m entryconf [$m index last] -state]
 		}
 	}
 }