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Mention that outdated versions may lead to build errors, and suggest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.

Don't write "[] " with an empty subject-prefix, but do write [v2] or
[M/N] if needed.

Add a --subject-prefix option that behaves roughly similarly to the
--subject-prefix option for git format-patch.

Signed-off-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>

We know the hash of the base commit, so provide that information as part
of the mail, in the same format format-patch uses.

Use "PATCH" rather than "PATCH 1/1" for that case.

The suggested command "git series -d" to delete the cover letter (when
all text of the cover is removed) doesn't work. It should be "git series
cover -d" instead.

Rebase has nothing to do in this case.

Notify the user but return successfully, so that it won't cause a rebase
to fail.

After parsing the revision, peel it to a commit before getting its id.

checkout detaches HEAD, but start left it attached, resulting in
inconsistent behavior.  Change start to detach HEAD as well.

Thanks to Michael Kerrisk for hosting a rendered version of the manpage.

Walk through developing or importing v1, and then developing v2 in
response to feedback.

Default to the behavior of git format-patch --from, which uses your
committer identity as the "From:" header for the patch mail, and
preserves commit authorship in an in-body "From:" header when formatting
commits you didn't write.  Add a --no-from option to use the commit
author as the "From:" address for each patch mail.

Clap normally distinguishes between "flags" (which don't take a value)
and "options" (which take a value), displaying them separately in help.
Enable the UnifiedHelpMessage setting, to unify these two in generated
help messages.

A series copied via "git branch git-series/newname git-series/oldname"
will not have any corresponding git-series-internals branches.  In that
case, checkout should populate those internals from the committed
version of the series.

In the process, since Internals::read_series now supports reading the
git-series ref as a fallback, change Internals::exists accordingly to
treat that as the series existing, and simplify its callers, who both
wanted that anyway.

Document the format of a git-series merge commit, which has multiple
git-series commits as parents in addition to the parents referencing
gitlinks.  (git-series does not yet generate such commits.)

Modify revision walking in "git series log" to walk a git-series merge
commit correctly.  ("git series log -p" will warn that it cannot show
diffs for such commits yet, as libgit2 doesn't support generating merge
diffs yet.)