Fix URLValidator to allow the '+' character in URL scheme

[Dheerendra Rathor]

Quoting w3's url-spec

A complete URL consists of a naming scheme specifier followed by a string whose format is a function of the naming scheme.

and

Within the URL of a object, the first element is the name of the scheme, separated from the rest of the object by a colon. The rest of the URL follows the colon in a format depending on the scheme.

So basically scheme should be present in URL syntax. Current scheme regex is ^(?:[a-z0-9\.\-]*):// which allows empty schemes.

Also schemes should allow alphanumeric characters with -, . and + (​rfc1738).

[Tim Graham]

When does a plus sign appear in the scheme in practice? I think schema validation is handled outside of the regular expression. Could you give an example of unexpected behavior with actual inputs?

[Dheerendra Rathor]

The very common example is git with ssh i.e. using git+ssh in scheme name. Hg-Git plugin uses this scheme ​http://hg-git.github.io/. Yeah they are somewhat rarely used. I guess Phabricator default ssh clone URL is also git+ssh.

[Tim Graham]

​PR

[Tim Graham <timograham@…>]

In 06627ef:

Fixed #25635 -- Made URLValidator allow '+' in scheme.