git-svn: ensure fetch always works chronologically.

git-svn: ensure fetch always works chronologically.

We run svn log against a URL without a working copy for the first fetch,
so we end up a log that's sorted from highest to lowest.  That's bad, we
always want lowest to highest.  Just default to --revision 0:HEAD now if
-r isn't specified for the first fetch.

Also sort the revisions after we get them just in case somebody
accidentally reverses the argument to --revision for whatever reason.

Thanks again to Emmanuel Guerin for helping me find this.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>