git-gui: Change rude error popup to info popup.

If the user has not added any files yet they cannot commit.  But
telling them this isn't an error, its really just an informational
note meant to push the user in the correct direction.

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

git-gui.sh

@@ -915,7 +915,7 @@ File [short_path $path] cannot be committed by this program.
 		}
 	}
 	if {!$files_ready} {
-		error_popup {No changes to commit.
+		info_popup {No changes to commit.
 
 You must add at least 1 file before you can commit.
 }