The function return value already provides the type.

Incorporates several performance improvements, and error handling.

This ensures that sorting by subject will put the patches in the right
order.

This removes a bit more duplication between diff and log -p, making it
easier to use diff elsewhere or further improve the format.

Inspired by tbdiff, the diff algorithm computes the interdiff for each
possible corresponding pair of commits (as well as the possibility of an
added or deleted commit), and then treats that as an instance of the
Assignment Problem to find a correspondence with the minimal total
interdiffs.

Add a new `git series diff` command to show the diff between the working
and staged versions of the patch series.  Also use the new diff format
for `git series log -p`.

Mention that outdated versions may lead to build errors, and suggest
downloading the stable version in that case as well.

Current versions of libgit2 no longer allow hiding commits while
walking.  With those versions, the revwalk would walk the entire project
history, not just the series history, taking seconds or minutes on large
repositories.  Work around that by doing the initial walk of series
commits by hand rather than with a revwalk.  The second walk still uses
a revwalk, for topological sorting.

This avoids code duplication; nothing wants to delete the internals
without also deleting the series.