annexInsteadOf config

annexInsteadOf config

Added config `url.<base>.annexInsteadOf` corresponding to git's
`url.<base>.pushInsteadOf`, to configure the urls to use for accessing the
git-annex repositories on a server without needing to configure
remote.name.annexUrl in each repository.

While one use case for this would be rewriting urls to use annex+http,
I decided not to add any kind of special case for that. So while
git-annex p2phttp, when serving multiple repositories, needs an url
of eg "annex+http://example.com/git-annex/ for each of them, rewriting an
url like "https://example.com/git/foo/bar" with this config set to
"https://example.com/git/" will result in eg
"annex+http://example.com/git-annex/foo/bar", which p2phttp does not
support.

That seems better dealt with in either git-annex p2phttp or a http
middleware, rather than complicating the config with a special case for
annex+http.

Anyway, there are other use cases for this that don't involve annex+http.