Populate $extensionPath if isset( $extension['path'] )

$extensionPath is initialized to null and then only set when no cached
vcs information was found. Instead set it to the proper value as soon as
we know that $extension['path'] exists. This will fix display of license
file links for most extensions.

Change-Id: I93bfedf3ae0cb7e6f9e077aa1d83906f7a3283b2
Merge "Populate $extensionPath if isset( $extension['path'] )" into wmf/1.24wmf13