Re: [PATCH] format-patch: teach format.useAutoBase "whenAble" option

Jacob Keller [off-list ref] writes:

From: Jacob Keller <redacted>

The format.useAutoBase configuration option exists to allow users to
enable '--base=auto' for format-patch by default.

This can sometimes lead to poor workflow, due to unexpected failures
when attempting to format an ancient patch:

    $ git format-patch -1 <an old commit>
    fatal: base commit shouldn't be in revision list

This can be very confusing, as it is not necessarily immediately obvious
that the user requested a --base (since this was in the configuration,
not on the command line).

We do want --base=auto to fail when it cannot provide a suitable base,
as it would be equally confusing if a formatted patch did not include
the base information when it was requested.

Teach format.useAutoBase a new mode, "whenAble". This mode will cause
format-patch to attempt to include a base commit when it can. However,
if no valid base commit can be found, then format-patch will continue
formatting the patch without a base commit. --base also learns the same
mode using the term "if-able".

Add tests to cover the new mode of operation for --base.

Two minor points.

 * would users get confused choosing between if-able and whenAble to
   use for the configuration and the command line option?

 * what if there is a branch called "if-able"?  The same problem
   already exists with a refname "auto", and this makes it worse.

I do not know what the best approach to solve the latter, but the
former would be easier to solve by just picking one and sticking to
it.

Thanks.