credential_format(): also encode <host>[:<port>]

An upcoming change wants to sanitize the credential password prompt
where a URL is displayed that may potentially come from a `.gitmodules`
file. To this end, the `credential_format()` function is employed.

To sanitize the host name (and optional port) part of the URL, we need a
new mode of the `strbuf_add_percentencode()` function because the
current mode is both too strict and too lenient: too strict because it
encodes `:`, `[` and `]` (which should be left unencoded in
`<host>:<port>` and in IPv6 addresses), and too lenient because it does
not encode invalid host name characters `/`, `_` and `~`.

So let's introduce and use a new mode specifically to encode the host
name and optional port part of a URI, leaving alpha-numerical
characters, periods, colons and brackets alone and encoding all others.

This only leads to a change of behavior for URLs that contain invalid
host names.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>