[BUG] url schemes should be case-insensitive

[Jeff King <peff@peff.net>]

We seem to match url schemes case-sensitively:

  $ git clone SSH://example.com/repo.git
  Cloning into 'repo'...
  fatal: Unable to find remote helper for 'SSH'

whereas rfc3986 is clear that the scheme portion is case-insensitive.
We probably ought to match at least our internal ones with strcasecmp.
Possibly we should also normalize external helpers (so "FOO://bar" would
always call git-remote-foo, never git-remote-FOO).

We could probably also give an advise() message in the above output,
suggesting that the problem is likely one of:

  1. They misspelled the scheme.

  2. They need to install the appropriate helper.

This may be a good topic for somebody looking for low-hanging fruit to
get involved in development (I'd maybe call it a #leftoverbits, but
since I didn't start on it, I'm not sure if it counts as "left over" ;)).

-Peff