Use bitmaps for non-commit reachability checks

Use bitmaps for non-commit reachability checks

Currently, unless RequestPolicy#ANY is used, UploadPack rejects all
non-commit "want" lines unless they were advertized. This is fine,
except when "uploadpack.allowreachablesha1inwant" is true
(corresponding to RequestPolicy#REACHABLE_COMMIT), in which case one
would expect that "want"-ing anything reachable would work.

(There is no restriction that "want" lines must only contain commits -
it is allowed for refs to directly point to trees and blobs, and
requesting for them using "want" lines works.)

This commit has been written to avoid performance regressions as much
as possible. In the usual (and currently working) case where the only
unadvertized things requested are commits, we do a standard RevWalk in
order to avoid incurring the cost of loading bitmaps. However, if
unadvertized non-commits are requested, bitmaps are used instead, and
if there are no bitmaps, a WantNotValidException is thrown (as is
currently done).

Change-Id: I68ed4abd0e477ff415c696c7544ccaa234df7f99
Signed-off-by: Jonathan Tan <jonathantanmy@google.com>