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.)

Fix the order of the definition to match other definitions, and fix
punctuation in the arg_from_usage call.

The commit message body will normally include a newline already.

Preserve write errors from write calls, and propagate them to the caller
of write_diff, rather than calling .unwrap().  This also avoids
attempting to propagate a panic through a libgit2 callback, which
re-panics.

This ensures that errors about the pager itself, such as broken pipes,
show up somewhere.

Add support for commands that support a pager but don't auto-pager
unless explicitly configured to do so.  This allows matching the
behavior of "git status" and "git branch", which don't auto-pager by
default.

Improve error handling in these commands to avoid panics on write errors
such as broken pipes.

After spawning the pager, if stderr would have gone to a tty, send it to
the pager as well.  That avoids having the pager and error output step
on each other.

This allows for interoperable implementations.