This only updates to Go 1.21 baseline, which is what the module still
specifies as the minimum version. I'll need to run this again when
moving to Go 1.25 or 1.26 as minimum.
This function parses patch headers (the preamble returned by the
existing Parse function) to extract information about the commit that
generated the patch. This is useful when patches are an interchange
format and this library is applying commits generated elsewhere.
Because of the variety of header formats, parsing is fairly lenient and
best-effort, although certain invalid input does cause errors.
This also extracts some test utilities from the apply tests for reuse.
Billy Keyes