mw-update-l10n: only copy when files differs

On beta cluster, MW_COMMON_SOURCE and MW_COMMON ends up pointing to the
same directory which is a NFS share.  That made the copy of
ExtensionMessages-master.php to fails because cp cant copy a file over
itself.

That broke l10nupdate for quite sometime. The workaround is to only copy
when MW_COMMON_SOURCE and MW_COMMON are different path by using readlink
to lookup the symbolic links.

Change-Id: I4944a0def72cdce85ddc6b6544652b49ec7da4f1