merge: expose multiple merge bases

We always calculate multiple merge bases, but up to now we had only
exposed the "best" merge base.

Introduce git_oidarray which analogously to git_strarray lets us return
multiple ids.
Merge pull request #2481 from libgit2/cmn/oidarray

merge: expose multiple merge bases
MSVC does not support zero size array
Merge pull request #2544 from linquize/test-push

MSVC does not support zero size array
transport: distinguish between unknown and unsupported transports

Even when built without a SSH support, we know about this transport. It
is implemented, but the current code makes us return an error message
saying it's not.

This is a leftover from the initial implementation of the transports
when there were in fact transports we knew about but were not
implemented.

Instead, let the SSH transport itself say it cannot run, the same as we
do for HTTPS.
transport: return ENOTFOUND for HTTPS and SSH when they're not supported

The previous commit makes it harder to figure out if the library was
built with support for a particular transport. Roll back some of the
changes and remove ssh:// and https:// from the list if we're being
built without support for them.
remote: get rid of git_remote_valid_url()

It does the same as git_remote_supported_url() but has a name which
implies we'd check the URL for correctness while we're simply looking at
the scheme and looking it up in our lists.

While here, fix up the tests so we check all the combination of what's
supported.