Use 5 MiB for RevWalk default limit

Use 5 MiB for RevWalk default limit

Instead of getting the limit from CoreConfig, use the larger of the
reader's limit or 5 MiB, under the assumption that any annotated tag
or commit of interest should be under 5 MiB.  But if a repository
was really insane and had bigger objects, the reader implementation
can set its streaming limit higher in order to allow RevWalk to
still process it.

Change-Id: If2c15235daa3e2d1f7167e781aa83fedb5af9a30
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>