UploadPack: Use reachable-sha1-in-want configuration

UploadPack: Use reachable-sha1-in-want configuration

C git 2.5 supports setting the equivalent of
RequestPolicy.REACHABLE_COMMIT with uploadpack.allowreachablesha1inwant.
Parse this into TransportConfig and use it from UploadPack. An explicitly
set RequestPolicy overrides the config, and the policy may still be
upgraded on a unidirectional connection to avoid races.

Change-Id: Id39771a6e42d8082099acde11249306828a053c0
Signed-off-by: Fredrik Medley <fredrik.medley@gmail.com>