git-svn: hopefully make 'fetch' more user-friendly

git-svn: hopefully make 'fetch' more user-friendly

multi-fetch is deprecated, "fetch -a" is easier to type
By default, fetch will fetch everything from its default
[svn-remote]; if fetch [--all|-a] is specified, then it will
fetch from all svn remotes.  Refspecs on the command-line
(like git-fetch) are not supported.

Also, enable -r/--revision arguments for fetch so
users can shoot themselves in the foot^W^W^W^W^W
skip some history and do the equivalent of a shallow
clone/fetch they're not interested in.

Signed-off-by: Eric Wong <normalperson@yhbt.net>