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>

strbuf.c

@@ -492,7 +492,9 @@ void strbuf_add_percentencode(struct strbuf *dst, const char *src, int flags)
 		unsigned char ch = src[i];
 		if (ch <= 0x1F || ch >= 0x7F ||
 		    (ch == '/' && (flags & STRBUF_ENCODE_SLASH)) ||
-		    strchr(URL_UNSAFE_CHARS, ch))
+		    ((flags & STRBUF_ENCODE_HOST_AND_PORT) ?
+		     !isalnum(ch) && !strchr("-.:[]", ch) :
+		     !!strchr(URL_UNSAFE_CHARS, ch)))
 			strbuf_addf(dst, "%%%02X", (unsigned char)ch);
 		else
 			strbuf_addch(dst, ch);