which we weren't treating right.
* $limit in wfDebugBacktrace() is the number of returned frames,
we thus need to take into account the wfDebugBacktrace() frame, which
is sliced from debug_backtrace().
* wfGetCaller() needs to add a level for itself.
* MWDebug::warning() was logging itself as the warning issuer,
which is useless (the call a few lines before was right, though)
MWDebugTest.php changed accordingly.
* Removed double call to wfGetCaller( $callerOffset + 1 )
* Documented the meaning of wfGetCaller() parameter
* Added unit test
Change-Id: Ief50f4c810bad8b03bb2bf9dc6d945d9acb29851
