remote: move relative_url()

This method was initially written in 63e95beb0 (submodule: port
resolve_relative_url from shell to C, 2016-05-15). As we will need
similar functionality in the bundle URI feature, extract this to be
available in remote.h.

The code is almost exactly the same, except for the following trivial
differences:

 * Fix whitespace and wrapping issues with the prototype and argument
   lists.

 * Let's call starts_with_dot_{,dot_}slash_native() instead of the
   functionally identical "starts_with_dot_{,dot_}slash()" wrappers
   "builtin/submodule--helper.c".

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

remote.c

@@ -14,6 +14,7 @@
 #include "strvec.h"
 #include "commit-reach.h"
 #include "advice.h"
+#include "connect.h"
 
 enum map_direction { FROM_SRC, FROM_DST };
 
@@ -2727,3 +2728,93 @@ void remote_state_clear(struct remote_state *remote_state)
 	hashmap_clear_and_free(&remote_state->remotes_hash, struct remote, ent);
 	hashmap_clear_and_free(&remote_state->branches_hash, struct remote, ent);
 }
+
+/*
+ * Returns 1 if it was the last chop before ':'.
+ */
+static int chop_last_dir(char **remoteurl, int is_relative)
+{
+	char *rfind = find_last_dir_sep(*remoteurl);
+	if (rfind) {
+		*rfind = '\0';
+		return 0;
+	}
+
+	rfind = strrchr(*remoteurl, ':');
+	if (rfind) {
+		*rfind = '\0';
+		return 1;
+	}
+
+	if (is_relative || !strcmp(".", *remoteurl))
+		die(_("cannot strip one component off url '%s'"),
+			*remoteurl);
+
+	free(*remoteurl);
+	*remoteurl = xstrdup(".");
+	return 0;
+}
+
+char *relative_url(const char *remote_url, const char *url,
+		   const char *up_path)
+{
+	int is_relative = 0;
+	int colonsep = 0;
+	char *out;
+	char *remoteurl = xstrdup(remote_url);
+	struct strbuf sb = STRBUF_INIT;
+	size_t len = strlen(remoteurl);
+
+	if (is_dir_sep(remoteurl[len-1]))
+		remoteurl[len-1] = '\0';
+
+	if (!url_is_local_not_ssh(remoteurl) || is_absolute_path(remoteurl))
+		is_relative = 0;
+	else {
+		is_relative = 1;
+		/*
+		 * Prepend a './' to ensure all relative
+		 * remoteurls start with './' or '../'
+		 */
+		if (!starts_with_dot_slash_native(remoteurl) &&
+		    !starts_with_dot_dot_slash_native(remoteurl)) {
+			strbuf_reset(&sb);
+			strbuf_addf(&sb, "./%s", remoteurl);
+			free(remoteurl);
+			remoteurl = strbuf_detach(&sb, NULL);
+		}
+	}
+	/*
+	 * When the url starts with '../', remove that and the
+	 * last directory in remoteurl.
+	 */
+	while (url) {
+		if (starts_with_dot_dot_slash_native(url)) {
+			url += 3;
+			colonsep |= chop_last_dir(&remoteurl, is_relative);
+		} else if (starts_with_dot_slash_native(url))
+			url += 2;
+		else
+			break;
+	}
+	strbuf_reset(&sb);
+	strbuf_addf(&sb, "%s%s%s", remoteurl, colonsep ? ":" : "/", url);
+	if (ends_with(url, "/"))
+		strbuf_setlen(&sb, sb.len - 1);
+	free(remoteurl);
+
+	if (starts_with_dot_slash_native(sb.buf))
+		out = xstrdup(sb.buf + 2);
+	else
+		out = xstrdup(sb.buf);
+
+	if (!up_path || !is_relative) {
+		strbuf_release(&sb);
+		return out;
+	}
+
+	strbuf_reset(&sb);
+	strbuf_addf(&sb, "%s%s", up_path, out);
+	free(out);
+	return strbuf_detach(&sb, NULL);
+}