Implement an advanced fuzzy diffing feature for interdiff

This implements a --fuzzy option to make interdiff perform a fuzzy
comparison between two diffs. This is very helpful, for example, for
comparing a backport patch to its upstream source patch to assist a human
reviewer in verifying the correctness of the backport.

The fuzzy diffing process is complex and works by:
- Generating a new patch file with hunks split up into smaller hunks to
  separate out multiple deltas (+/- lines) in a single hunk that are spaced
  apart by context lines, increasing the amount of deltas that can be
  applied successfully with fuzz
- Applying the rewritten p1 patch to p2's original file, and the rewritten
  p2 patch to p1's original file; the original files aren't ever merged
- Relocating patched hunks in only p1's original file to align with their
  respective locations in the other file, based on the reported line
  offset printed out by `patch` for each hunk it successfully applied
- Squashing unline gaps fewer than max_context*2 lines between hunks in the
  patched files, to hide unknown contextual information that is irrelevant
  for comparing the two diffs while also improving hunk alignment between
  the two patched files
- Diffing the two patched files as usual
- Rewriting the hunks in the diff output to exclude unlines from the
  unified diff, even splitting up hunks to remove unlines present in the
  middle of a hunk, while also adjusting the @@ line to compensate for the
  change in line offsets
- Emitting the rewritten diff output while interleaving rejected hunks from
  both p1 and p2 in the output in order by line number, with a comment on
  the @@ line indicating when an emitted hunk is a rejected hunk

This also involves working around some bugs in `patch` itself encountered
along the way, such as occasionally inaccurate line offsets printed out and
spurious fuzzing in certain cases that involve hunks with an unequal number
of pre-context and post-context lines.

The end result of all of this is a minimal set of real differences in the
context lines of each hunk between the user's provided diffs. Even when
fuzzing results in a faulty patch, the context differences are shown so
there is never a risk of any real deltas getting hidden due to fuzzing.

By default, the fuzz factor used is just the default used in `patch`. The
fuzz factor can be adjusted by the user via appending =N to `--fuzzy` to
specify the maximum number of context lines for `patch` to fuzz.

Author: kerneltoast