[RFC] git-svn: make git-svn commit-diff able to work without explicit arguments

Hello,

When using git-svn to access a SVN repo, the commit policy may vary. 
While git makes you commit small patches often, svn users tend to prefer 
bigger patches that implement a functionnality at once.

So at the end you have a SVN commit which corresponds to several git ones.

What you can do in this case is :

   git-svn commit-diff --edit -r$REV remotes/git-svn HEAD

Which effect is that it commits (at once) all the commits between the 
latest svn fetch and HEAD.

What I'm proposing here is this:

  - use the latest fetched rev the default for the -r argument.
  - use remotes/git-svn and HEAD the defaults for the treeish objects.

A smarter way to take these defaults would be to take the last revision 
in the current branch (which can be something else than git-svn if it 
wasn't rebased/merged recently) and the relevant commit in the current 
branch.

Additionnaly, --edit could be enabled by default if -m is not set and it 
is used interactively, eventually using an option in repo-config.

Any comment ?