Perhaps we want {diff,log,format}.noprefix and friends?

[Junio C Hamano <gitster@pobox.com>]

>>   git -c diff.noprefix=no format-patch ...
>>
>> to cancel the config. (If that config really does want to be on, that
>> is.)
>>
>> That said, something like
>>
>>   git am -p0 ...
>>
>> should help on the receiving side, by way of skipping fewer path
>> components when applying the patch.

While it is very reasonable for end-users to make the output of
their local "git diff" output to their taste by setting the
diff.noprefix and other configuration options in the diff.*
namespace, it is wrong to inflict such a personal preference on
other project participants by sending such a no-prefix patch via
"git format-patch", when the convention at the receiving project is
to use "git am -p1" to accept them.

We would need to tweak the configuration system so that we can more
easily introduce "format.noprefix" that overrides "diff.noprefix"
only when the command being run is "git format-patch", perhaps?  

As things stand, we could teach builtin/log.c::git_format_config()
about "format.noprefix" and make "git format-patch" pay attention to
that in addition to "diff.noprefix" fairly easily, but such an
approach poorly scales.  When a new configuration gets introduced in
diff.c::git_diff_ui_config(), builtin/log.c::git_format_config()
would also need to be updated.

I initially started writing this with "left over bits" mark, but it
seems it is a bit larger in scope than that.

I also noticed that some format.* configuration variables are parsed
in git_log_config(), not git_format_config(), so "git log" would end
up honoring format.pretty and format.subjectprefix, etc., which look
wrong but probably is way too late to "fix".