Add GitViewers hook for extensions to add git viewers to SpecialVersion

Add GitViewers hook for extensions to add git viewers to SpecialVersion

This adds a GitViewers hook to GitInfo, and slightly refactors the
$viewers variable to be called through a function so the hook can be
called. This is so extensions can add their own git repository viewer
to the Special:Version git sha field.

Change-Id: Ifc8396984de6a51be85fb63cd9754d57333bde03