git-svn: add a 'rebase' command

git-svn: add a 'rebase' command

This works similarly to 'svn update' or 'git pull' except that
it preserves linear history with 'git rebase' instead of 'git
merge' for ease of dcommit-ing with git-svn.

While we're at it, put the working_head_info() logic
into its own function and allow --fetch-all/--all for
dcommit and rebase (which will fetch all refs in the
current [svn-remote] instead of just the working one).

Note that the '-a' switch (short for --fetch-all/--all) has been
removed as it conflicts with the non-svn 'git fetch'

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