format.outputDirectory should be based on the top of the repository

[Denton-L]

Currently, if one has format.outputDirectory set, the output directory will be relative to wherever they've run the command, not the top of the repository. We should make it relative to the Git repository instead so that users don't end up creating patches in unintended locations.

Or maybe the current behaviour is desired and we should just force users to use an absolute path if they don't want this to happen. This requires discussion.

[Denton-L]

An experimental implementation of this: https://github.com/Denton-L/git/tree/experimental/format-output-relative-gitdir

[Denton-L]

Submission here: https://public-inbox.org/git/9422e4225143b2b908bd1fed9a510b9333bf34a2.1577191015.git.liu.denton@gmail.com/