git-gui: use commit message template

Use the file described by commit.template (if set) to show the commit message
template, just like other GUIs.

Signed-off-by: Martin Schön <Martin.Schoen@loewensteinmedical.de>
Signed-off-by: Pratyush Yadav <me@yadavpratyush.com>
diff --git a/lib/commit.tcl b/lib/commit.tcl
index b516aa2..11379f8 100644
--- a/lib/commit.tcl
+++ b/lib/commit.tcl
@@ -456,6 +456,7 @@
 	}
 
 	$ui_comm delete 0.0 end
+	load_message [get_config commit.template]
 	$ui_comm edit reset
 	$ui_comm edit modified false
 	if {$::GITGUI_BCK_exists} {