in_merge_bases(): support only one "other" commit

in_merge_bases(): support only one "other" commit

In early days of its life, I planned to make it possible to compute
"is a commit contained in all of these other commits?" with this
function, but it turned out that no caller needed it.

Just make it take two commit objects and add a comment to say what
these two functions do.

Signed-off-by: Junio C Hamano <redacted>