Add config option for git-fetch refs hack

Add config option for git-fetch refs hack

New config file option 'repo-cache-refs-hack' for enabling the refs hack
in 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 may
encountered with 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: I5e0203d4f0f93c18c2f70c76ef9c8b6ff536fd67
Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>