urlmatch: add underscore to URL_HOST_CHARS

urlmatch: add underscore to URL_HOST_CHARS

When parsing a URL to normalize it, we allow hostnames to contain only
dot (".") or dash ("-"), plus brackets and colons for IPv6 literals.
This matches the old URL standard in RFC 1738, which says:

  host           = hostname | hostnumber
  hostname       = *[ domainlabel "." ] toplabel
  domainlabel    = alphadigit | alphadigit *[ alphadigit | "-" ] alphadigit

But this was later updated by RFC 3986, which is more liberal:

  host        = IP-literal / IPv4address / reg-name
  reg-name    = *( unreserved / pct-encoded / sub-delims )
  unreserved  = ALPHA / DIGIT / "-" / "." / "_" / "~"

While names with underscore in them are not common and possibly violate
some DNS rules, they do work in practice, and we will happily contact
them over http://, git://, or ssh://. It seems odd to ignore them for
purposes of URL matching, especially when the URL RFC seems to allow
them.

There shouldn't be any downside here. It's not a syntactically
significant character in a URL, so we won't be confused about parsing;
we'd have simply rejected such a URL previously (the test here checks
the url code directly, but the obvious user-visible effect would be
failing to match credential.http://foo_bar.example.com.helper, or
similar config in http.<url>.*).

Arguably we'd want to allow tilde ("~") here, too. There's likewise
probably no downside, but I didn't add it simply because it seems like
an even less likely character to appear in a hostname.

Reported-by: Alex Waite <alex@waite.eu>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>