UploadPack: support deepen-since in protocol v2

UploadPack: support deepen-since in protocol v2

Support the deepen-since parameter when requested by a client using
protocol v2. This is done by:
 - adding a DepthWalk.RevWalk#setDeepenSince method
 - updating DepthGenerator to recognize when deepen-since is set
 - recording in DepthWalk.Commit whether a commit is a boundary commit

Existing users of DepthWalk such as UploadPack previously recognized
boundary commits by comparing their depths against the threshold, not
tracking whether any parents were truly excluded. This behavior is
preserved - UploadPack considers a commit as boundary if its depth is
equal to the threshold *or* a parent was excluded (whether by depth or
by deepen-since).

Change-Id: I852bba6b1279f9cc8aee38282e9339d62b8dcddc
Signed-off-by: Jonathan Tan <jonathantanmy@google.com>