git svn: handle errors and concurrent commits in dcommit

git svn: handle errors and concurrent commits in dcommit

dcommit didn't handle errors returned by SVN and coped very
poorly with concurrent commits that appear in SVN repository
while dcommit was running. In both cases it left git repository
in inconsistent state: index (which was reset with `git reset
--mixed' after a successful commit to SVN) no longer matched the
checkouted tree, when the following commit failed or needed to be
rebased. See http://bugs.debian.org/676904 for examples.

This patch fixes the issues by:
- introducing error handler for dcommit. The handler will try
  to rebase or reset working tree before returning error to the
  end user. dcommit_rebase function was extracted out of cmd_dcommit
  to ensure consistency between cmd_dcommit and the error handler.
- calling `git reset --mixed' only once after all patches are
  successfully committed to SVN. This ensures index is not touched
  for most of the time of dcommit run.

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