UploadPack: Use reachability checker to validate non-advertised wants

UploadPack: Use reachability checker to validate non-advertised wants

In "Reachable commit" request validators, we need to check that a "want"
in the request, that hasn't been advertised, is reachable from the refs
visible to the user.

Current code has intermixed the translation of ObjectIds to RevCommits
(and its error handling) with the actual walk, with the delegation to
bitmaps in restricted circunstances.

Refactor the code to make it "flatter" and more readable. Move ObjectIds
to RevCommits translation to its own functions. Use the reachability
checker instead of a newly defined walk.

Before the non-advertised wants were validated with bitmaps only if any
"want" refered to an non-commit. Now they will be validated with bitmaps
also if the "wants" refer all to commits.

Change-Id: Ib925a48cde89672b07a88bba4e24d0457546baff
Signed-off-by: Ivan Frade <ifrade@google.com>