Like `git_patch_to_buf`, provide a simple helper method that can
print an entire diff directory to a `git_buf`.

Parse diff files into a `git_diff` structure.

Now that `git_diff_delta` data can be produced by reading patch
file data, which may have an abbreviated oid, allow consumers to
know that the id is abbreviated.

When formatting a patch as email we do not include the commit's
message in the formatted patch output. Implement this and add a
test that verifies behavior.

Now that non-pathspec matching diffs are implemented at the iterator
level, drop `FILELIST_MATCH`ing.

Document that `GIT_DIFF_PATHSPEC_DISABLE` is not necessarily about
explicit path matching, but also includes matching of directory
names.  Enforce this in a test.