Add svn-compatible "blame" output format to git-svn

Add svn-compatible "blame" output format to git-svn

git-svn blame produced output in the format of git blame; in environments
where there are scripts that read the output of svn blame, it's useful
to be able to use them with the output of git-svn. The git-compatible
format is still available using the new "--git-format" option.

This also fixes a bug in the initial git-svn blame implementation; it was
bombing out on uncommitted local changes.

Signed-off-by: Steven Grimm <koreth@midwinter.com>
Acked-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>