[PATCH 0 of 2] Introduce flags for Shallow Clones

[Vishakh H]

The following patches introduce the REVLOGSHALLOW header flag and
REVIDX_PUNCHED_FLAG & REVIDX_KNOWN_FLAGS revision flags.

The header flag is useful to relax constraints and perform other actions
on the revlog when it identifies itself as shallow. The punched flag is
used for any revision where no useful data is available, in the case of
shallow clones when an unwanted parent is introduced to the index and
its data and parents are null.