prefix debug log when they end up in the main log

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