git svn: fix dcommit to work with touched files

git svn: fix dcommit to work with touched files

The dcommit command fails if an otherwise unmodified file has
been touched in the working directory:

    Cannot dcommit with a dirty index.  Commit your changes
    first, or stash them with `git stash'.

This happens because "git diff-index" reports a difference
between the index and the filesystem:

    :100644 100644 d00491...... 000000...... M      file

The fix is to run "git update-index --refresh" before
"git diff-index" as is done in git-rebase and
git-rebase--interactive before "git diff-files".

This changes dcommit to display a list of modified files before
exiting.

Also add a similar test case for "git svn rebase".

[ew: rearranged commit message subject]

Signed-off-by: David D. Kilzer <ddkilzer@kilzer.net>
Acked-by: Eric Wong <normalperson@yhbt.net>