Enable gerrit/gitlab/github refspecs

By default git only fetches references under the refs/heads/ tree,
this patch adds support to kas to enable you to specify references
outside of the ref/heads tree.  This is useful as it allows you to
use uncommitted gerrit patchsets, Gitlab merge requests or github
pull requests that live under refs/changes/, refs/merge-requests
and refs/pull as the reference for a repo allowing the use of
in development changes.  When a refsepc is defined that starts
with refs/ an additional git fetch operation is preformed on the
repo to explicitly fetch the reference given so it can be checked
out for use.

Signed-off-by: Drew Reed <drew.reed@arm.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

tests/test_refspec/test3.yml

@@ -0,0 +1,13 @@
+header:
+  version: 8
+
+repos:
+  this:
+
+  kas_abs:
+    url: https://github.com/siemens/kas.git
+    refspec: refs/heads/master
+
+  kas_rel:
+    url: https://github.com/siemens/kas.git
+    refspec: master