git-svn: fix dcommit losing changes when out-of-date from svn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silently clobbered by git-svn if our repository is out-of-date.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/Documentation/git-svn.txt b/Documentation/git-svn.txt
index 450ff1f..a764d1f 100644
--- a/Documentation/git-svn.txt
+++ b/Documentation/git-svn.txt
@@ -120,6 +120,7 @@
 	URL of the target Subversion repository.  The final argument
 	(URL) may be omitted if you are working from a git-svn-aware
 	repository (that has been init-ed with git-svn).
+	The -r<revision> option is required for this.
 
 'graft-branches'::
 	This command attempts to detect merges/branches from already