Add 'resolve_gitlink_ref()' helper function

Add 'resolve_gitlink_ref()' helper function

This new function resolves a ref in *another* git repository.  It's
named for its intended use: to look up the git link to a subproject.

It's not actually wired up to anything yet, but we're getting closer to
having fundamental plumbing support for "links" from one git directory
to another, which is the basis of subproject support.

[jc: amended a FILE* leak]

Signed-off-by: Linus Torvalds <redacted>
Signed-off-by: Junio C Hamano <redacted>