Change RevObject dispose() semantics to avoid reparses

Change RevObject dispose() semantics to avoid reparses

UploadPack was suffering from performance problems because dispose()
was unsetting the PARSED flag, causing another visit to a commit to
reparse the object from disk, even though we already had its parents
in memory.  If a client was 50,000 commits ahead of the server,
then the client could send 50,000 have requests, each time the
server is executing a full reparse of its own commit graph trying
to determine if the common set is sufficient yet.

Instead of asking applications to dispose of commit buffers when the
commit is popped out of the RevWalk we now dispose of the buffer as
soon as the headers are parsed.  For applications that never need
the buffer but have to process a lot of commits in the pending queue,
this can reduce memory usage by a fair amount, as the pending queue
no longer has to retain the buffers until the commits are returned
to the application for buffer disposal.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Robin Rosenberg <robin.rosenberg@dewire.com>