git-svn: fix dcommit clobbering when committing a series of diffs

git-svn: fix dcommit clobbering when committing a series of diffs

Our revision number sent to SVN is set to the last revision we
committed if we've made any previous commits in a dcommit
invocation.

Although our SVN Editor code uses the delta of two (old) trees
to generate information to send upstream, it'll still send
complete resultant files upstream; even if the tree they're
based against is out-of-date.

The combination of sending a file that does not include the
latest changes, but set with a revision number of a commit we
just made will cause SVN to accept the resultant file even if it
was generated against an old tree.

More trouble was caused when fixing this because we were
rebasing uncessarily at times.  We used git-diff-tree to check
the imported SVN revision against our HEAD, not the last tree we
committed to SVN.  The unnecessary rebasing caused merge commits
upstream to SVN to fail.

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