libgit2: fix for broken SSH host callback

[StefanKarpinski]

This PR fixes a couple of issues with #39324 as reported in #38777:

1. The patch caused libgit2 to crash when cloning with a user@host implicit SSH URL (as opposed to an explicit ssh://user@host URL which worked correctly).
2. The callback was passing a byte vector when the method expected a string; this changes the method to take a byte vector instead.

[jmert]

I can confirm this fixes both issues for me. Thanks!

[maleadt]

Backport? I ran into the ssh_knownhost_check MethodError on the backports-release-1.6 branch. cc @KristofferC

[fxcoudert]

This patch introduces a bug:

if (atoi(urldata.port) == SSH_DEFAULT_PORT) {

That condition can never be true because in libgit2 we have:

#define SSH_DEFAULT_PORT "22"

so SSH_DEFAULT_PORT is a pointer, not an integer value.