Re: [PATCH v3 06/10] diff-lib: define diff_get_merge_base()

Denton Liu [off-list ref] writes:

became something like

	if (diff_get_merge_base(revs, &oid))
		exit(128);

so I do agree with the spirit of the change but in reality, it just
creates more busywork for the callers.

OK, then lets keep them as die()s.

I think that it should be die-worthy because it's a logic error for a
user to do this. I can't think of any situation where it wouldn't be
more desirable error early to correct a user's thinking. Plus, we're
trying to move away from the `...` notation anyway ;)

I do not think so.  We are *NOT* trying to move away from A...B;
what is mistake is A..B and that is what we want to move away from.
Luckily, there is no need to introduce a new option there, because
the user can just stop typing .. and instead type SP.

The primary value of the new option you are adding is that it allows
us to compare the index and the working tree with the merge base.
The current A...B notation can only be used to compare two trees.