prefix debug log when they end up in the main log

Whenever we use wfDebugLog( "group", "message" ); and "group" is not
also set in $wfDebugLogGroups, the group information is lost. By
prefixing the message with the group name, it makes it obvious where the
message comes from.

Example output:

...
[EventLogging] wgEventLoggingSchemaIndexUri has not been configured.
[EventLogging] EventLogging requires memcached, and no memcached[...]
Fully initialised
Profiler::instance called without $wgProfiler['class'] set[...]
Connected to database 0 at
MessageGroups::init-nocache
SQL ERROR (ignored): no such table: revtag_type
MessageGroups::loadGroupDefinitions-end
[ContentHandler] Created handler for wikitext: WikitextContentHandler
[ContentHandler] Created handler for javascript: JavaScriptContentHandler
...

Change-Id: Id75401bec516b5111cd1e3d4a9b29843953ce12b

RELEASE-NOTES-1.21

@@ -57,6 +57,9 @@ production.
   debug mode: wfMsg, wfMsgNoTrans, wfMsgForContent, wfMsgForContentNoTrans,
   wfMsgReal, wfMsgGetKey, wfMsgHtml, wfMsgWikiHtml, wfMsgExt, wfEmptyMsg. Use
   the Message class, or the global method wfMessage.
+* Debug message emitted by wfDebugLog() will now be prefixed with the group
+  name when its logged to the default log file. That is the case whenever the
+  group has no key in wgDebugLogGroups, that will help triage the default log.
 
 === Bug fixes in 1.21 ===
 * (bug 40353) SpecialDoubleRedirect should support interwiki redirects.

includes/GlobalFunctions.php

@@ -1037,7 +1037,7 @@ function wfDebugLog( $logGroup, $text, $public = true ) {
 			wfErrorLog( "$time $host $wiki: $text", $wgDebugLogGroups[$logGroup] );
 		}
 	} elseif ( $public === true ) {
-		wfDebug( $text, true );
+		wfDebug( "[$logGroup] $text", true );
 	}
 }