* Document that settting preferred content to "" is the same as the
default unset behavior.
* sync: Avoid misleading warning about future preferred content
transition when preferred content is set to "".
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.
Work around git hash-object --stdin-paths's odd stripping of carriage
return from the end of the line (some windows infection), avoiding crashing
when the repo contains a filename ending in a carriage return.
Make programPath never return "git-remote-annex" or other known multi-call
program names, which are not git-annex and won't behave like it.
If the git-annex binary gets installed under some entirely other name,
it will still return it.
This change exposed that readProgramFile actually could crash,
which happened before only if getExecutablePath was not absolute
and there was no ~/.config/git-annex/program. So fixed that to catch
exception.
