remote: apply insteadOf configuration.

A remote's URLs are now modified according to the url.*.insteadOf
and url.*.pushInsteadOf configurations. This allows a user to
replace URL prefixes by setting the corresponding keys. E.g.
"url.foo.insteadOf = bar" would replace the prefix "bar" with the
new prefix "foo".

include/git2/remote.h

@@ -120,6 +120,9 @@ GIT_EXTERN(const char *) git_remote_name(const git_remote *remote);
 /**
  * Get the remote's url
  *
+ * If url.*.insteadOf has been configured for this URL, it will
+ * return the modified URL.
+ *
  * @param remote the remote
  * @return a pointer to the url
  */
@@ -128,6 +131,9 @@ GIT_EXTERN(const char *) git_remote_url(const git_remote *remote);
 /**
  * Get the remote's url for pushing
  *
+ * If url.*.pushInsteadOf has been configured for this URL, it
+ * will return the modified URL.
+ *
  * @param remote the remote
  * @return a pointer to the url or NULL if no special url for pushing is set
  */