Fix suggestions algorithm

[pbrisbin]

At its core, getting suggestions from a patch involves looking for contiguous blocks of
either removes, or removes-then-adds. The former would be a suggestion to remove
some content, and the latter would be a suggestion to replace it.

The old version of this algorithm used line numbers to look for contiguous blocks: if
a line's number was the last line + 1, it was considered contiguous. This is wildly
incorrect since removed and added lines' numbers increment independently.

This was working fine, which made all our tests pass:

1 1 # a line
2   # remove this
  2 # add this

But this example breaks:

1 1 # a line
2   # remove this
3 2
4 3 # more
5   # remove this
  4 # add this

Notice how 4 (on the add side) should be considered as contiguous with 5 (on the
remove side). Obviously, looking for line numbers based on previous line numbers
won't work.

The new algorithm groups lines of equal type (add, remove, context)¹ and also
considers an add immediately following a remove as equal. This turns a diff like
the above into the kind of groupings we'd visualize when looking for suggestions,

---
1 1 # a line
---
2   # remove this
---
3 2
4 3 # more
---
5   # remove this
  4 # add this
---

Once grouped, any group that has removes is a suggestion. If it also has adds, that's used
otherwise it's a suggested removal.

Footnotes

1. This, sadly, meant basically re-implementing parse-git-patch ourselves. ↩