git_get_ref: Move to Dgit.pm and reimplement

Use
  git for-each-ref --format=%(objectname) [r]efs/whatever

This pattern won't match literally as a prefix because a ref name
cannot contain [, so it must match according to fnmatch.  whatever
cannot contain fnmatch metacharacters because they're not valid in ref
names.

This is a better idiom because it might directly look up the ref in
question (although currently it probably doesn't).