contrib/git-svn: several small bug fixes and changes

contrib/git-svn: several small bug fixes and changes

 * Fixed manually-edited commit messages not going to
   remotes/git-svn on sequential commits after the sequential
   commit optimization.
 * format help correctly after adding 'show-ignore'
 * sha1_short regexp matches down to 4 hex characters
   (from git-rev-parse --short documentation)
 * Print the first line of the commit message when we commit to
   SVN next to the sha1.
 * Document 'T' (type change) in the comments

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