revision.c: add BOTTOM flag for commits

revision.c: add BOTTOM flag for commits

When performing edge-based operations on the revision graph, it can be
useful to be able to identify the INTERESTING graph's connection(s) to
the bottom commit(s) specified by the user.

Conceptually when the user specifies "A..B" (== B ^A), they are asking
for the history from A to B. The first connection from A onto the
INTERESTING graph is part of that history, and should be considered. If
we consider only INTERESTING nodes and their connections, then we're
really only considering the history from A's immediate descendants to B.

This patch does not change behaviour, but adds a new BOTTOM flag to
indicate the bottom commits specified by the user, ready to be used by
following patches.

We immediately use the BOTTOM flag to return collect_bottom_commits() to
its original approach of examining the pending commit list rather than
the command line. This will ensure alignment of the definition of
"bottom" with future patches.

Signed-off-by: Kevin Bracey <kevin@bracey.fi>
Signed-off-by: Junio C Hamano <gitster@pobox.com>