commit-reach: introduce merge_base_flags enum

commit-reach: introduce merge_base_flags enum

Replace the boolean ignore_missing_commits parameter in
paint_down_to_common() with an enum merge_base_flags, and thread
the flags through merge_bases_many(), get_merge_bases_many_0(),
and the public repo_get_merge_bases_many_dirty() API.

This makes callsites with boolean parameters easier to read and
prepares the function for additional flags in a subsequent commit.

No functional change: the single caller that used
ignore_missing_commits (repo_in_merge_bases_many) now sets
MERGE_BASE_IGNORE_MISSING_COMMITS in the flags word, and all
other callers pass 0.

Signed-off-by: Kristofer Karlsson <krka@spotify.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>