Support protocol v2 want-ref in UploadPack

Support protocol v2 want-ref in UploadPack

UploadPack already allows the client to send wanted OIDs as "want"
lines. Extend UploadPack to also allow the client to send wanted ref
names as "want-ref" lines when the fetch is done using protocol v2.

The corresponding Git commit is 516e2b76bd ("upload-pack: implement
ref-in-want", 2018-06-28).

To support a two-stage rollout, two configuration variables are
provided: uploadpack.allowrefinwant (default "false") allows clients to
specify "want-ref" in their requests, and uploadpack.advertiserefinwant
(default "true") makes UploadPack advertise this capability. If
uploadpack.allowrefinwant is true but uploadpack.advertiserefinwant is
false, UploadPack will not advertise that it supports "want-ref", but it
will support it.

Change-Id: I3c24077949640d453af90d81a7f48ce4b8ac9833
Signed-off-by: Jonathan Tan <jonathantanmy@google.com>