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.