This is a significant reorganization of the diff code to break it
into a set of more clearly distinct files and to document the new
organization. Hopefully this will make the diff code easier to
understand and to extend.
This adds a new `git_diff_driver` object that looks of diff driver
information from the attributes and the config so that things like
function content in diff headers can be provided. The full driver
spec is not implemented in the commit - this is focused on the
reorganization of the code and putting the driver hooks in place.
This also removes a few #includes from src/repository.h that were
overbroad, but as a result required extra #includes in a variety
of places since including src/repository.h no longer results in
pulling in the whole world.
This adds tests for diffs with submodules in them and (perhaps
unsurprisingly) requires further fixes to be made. Specifically,
this fixes:
- when considering if a submodule is dirty in the workdir, it was
being treated as dirty even if only the index was dirty.
- git_diff_patch_to_str (and git_diff_patch_print) were "printing"
the headers for files (and submodules) that were unmodified or
had no meaningful content.
- added comment to previous fix and removed unneeded parens.
