Add config option for repocache git-fetch refs hack

Add config option for repocache git-fetch refs hack

New config file option 'repo-cache-refs-hack' for enabling refs hack in
gbp-repocache. The option is disabled, by default.

Enabling the hack makes it possible to fetch/clone remote repositories
that have branches (refs/heads/*) that point to tag objects. This is
possible in Gerrit repositories - "pure" Git repositories managed
by Git tools shouldn't have such branches as Git sanity checking
prevents them (which is the reason this hack is needed).

Change-Id: Ic3a1814975f3c11d6c01734416f64347b9c73986
Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>