git-gui: added config gui.gcwarning to disable the gc hint message

On startup in multicommit mode git-gui checks to see if the repository
has a lot of objects. If so it shows a dialog suggesting gc be run.
This adds 'gui.gcwarning' as a control config variable to allow this
to be disabled. The default is true (the warning is shown). Setting this
false will prevent the check being done.
Signed-off-by: NPat Thoyts <patthoyts@users.sourceforge.net>

git-gui.sh

@@ -3894,7 +3894,7 @@ after 1 {
 		$ui_comm configure -state disabled -background gray
 	}
 }
-if {[is_enabled multicommit]} {
+if {[is_enabled multicommit] && ![is_config_false gui.gcwarning]} {
 	after 1000 hint_gc
 }
 if {[is_enabled retcode]} {