credential: convert "url" attribute into its parsed subparts

The git-credential command requires that you feed it a
broken-down credential, which means that the client needs to
parse a URL itself. Since we have our own URL-parsing
routines, we can easily allow the caller to just give us the
URL as-is, saving them some code.

Signed-off-by: Jeff King <peff@peff.net>
Acked-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-credential.txt

@@ -140,3 +140,15 @@ Git understands the following attributes:
 `password`::
 
 	The credential's password, if we are asking it to be stored.
+
+`url`::
+
+	When this special attribute is read by `git credential`, the
+	value is parsed as a URL and treated as if its constituent parts
+	were read (e.g., `url=https://example.com` would behave as if
+	`protocol=https` and `host=example.com` had been provided). This
+	can help callers avoid parsing URLs themselves.  Note that any
+	components which are missing from the URL (e.g., there is no
+	username in the example above) will be set to empty; if you want
+	to provide a URL and override some attributes, provide the URL
+	attribute first, followed by any overrides.

credential.c

@@ -172,6 +172,8 @@ int credential_read(struct credential *c, FILE *fp)
 		} else if (!strcmp(key, "path")) {
 			free(c->path);
 			c->path = xstrdup(value);
+		} else if (!strcmp(key, "url")) {
+			credential_from_url(c, value);
 		}
 		/*
 		 * Ignore other lines; we don't know what they mean, but