commit-reach(repo_in_merge_bases_many): report missing commits

commit-reach(repo_in_merge_bases_many): report missing commits

Some functions in Git's source code follow the convention that returning
a negative value indicates a fatal error, e.g. repository corruption.

Let's use this convention in `repo_in_merge_bases()` to report when one
of the specified commits is missing (i.e. when `repo_parse_commit()`
reports an error).

Also adjust the callers of `repo_in_merge_bases()` to handle such
negative return values.

Note: As of this patch, errors are returned only if any of the specified
merge heads is missing. Over the course of the next patches, missing
commits will also be reported by the `paint_down_to_common()` function,
which is called by `repo_in_merge_bases_many()`, and those errors will
be properly propagated back to the caller at that stage.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>