RFC: indicating diff strategy in format-patch message headers

[Konstantin Ryabitsev <konstantin@linuxfoundation.org>]

Hello, all:

When developing tooling that attempts to map commits to patches, we often
don't have more than just the git-patch-id to go by. The problem is, there is
any number of ways to generate patches from commits:

- using a different strategy (--histogram, --patience, etc)
- using a different number of context lines (-U5)

Without knowing what options were used by the original author, we cannot be
certain that we'll create a patch with the same git-patch-id, even if it's
from the exact same commit.

Would it make sense to have git-format-patch (and friends) include an
additional header hinting at the options used to generate the patch? E.g.:

    X-git-diff-options: algo=myers; context=3;

It won't help for all cases where we need to make a match (e.g. when we match
from git commits to a patch query), but it will help matching the other way.

Any thoughts?

-K