git-svn: fix dcommit losing changes when out-of-date from svn

git-svn: fix dcommit losing changes when out-of-date from svn

There was a bug in dcommit (and commit-diff) which caused deltas
to be generated against the latest version of the changed file
in a repository, and not the revision we are diffing (the tree)
against locally.

This bug can cause recent changes to the svn repository to be
silently clobbered by git-svn if our repository is out-of-date.

Thanks to Steven Grimm for noticing the bug.

The (few) people using the commit-diff command are now required
to use the -r/--revision argument.  dcommit usage is unchanged.

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