git-svn: fix dcommit losing changes when out-of-date from svn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silently clobbered by git-svn if our repository is out-of-date.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>
diff --git a/t/t9105-git-svn-commit-diff.sh b/t/t9105-git-svn-commit-diff.sh
index f994b72..746c827 100755
--- a/t/t9105-git-svn-commit-diff.sh
+++ b/t/t9105-git-svn-commit-diff.sh
@@ -33,7 +33,7 @@
 
 test_expect_success 'test the commit-diff command' "
 	test -n '$prev' && test -n '$head' &&
-	git-svn commit-diff '$prev' '$head' '$svnrepo' &&
+	git-svn commit-diff -r1 '$prev' '$head' '$svnrepo' &&
 	svn co $svnrepo wc &&
 	cmp readme wc/readme
 	"