"bzr send" misuses the .patch extension

[Xavier (Open ERP)]

"bzr send" consistently generates bundles with the ".patch" extension (leading to an x-diff mime type) even when --no-patch is used (meaning the bundle cannot be used as a patch). This extension can *not* be overridden as specifying an output file is equivalent to using bzr bundle instead (doesn't send an e-mail, only generates a file).

This has two problematic consequences:
* If the file isn't generated with --no-patch and the accompanying email doesn't clearly specify that the payload is a bazaar bundle file (as well as the instructions to merge the payload into an existing branch), it's usually going to be applied with `patch`, losing any revision information and all binary data (e.g. images) in the process
* If the file is generated with --no-patch and the accompanying email doesn't clearly specify that the payload is a bazaar bundle file, the destination finds himself with a "corrupted" patch file full of binary gunk.

Solutions:
1. Change the extension to a specific one (e.g. `.bzrbundle`) at least when using --no-patch as the result can *not* be used as a patchfile, let the user force a non-`.patch` extension even when not using --no-patch (or let the user specify his own filename/extension for the output file)
2. Ideally, bazaar should be able to send revisions using the model mercurial and git use: a sequence of plain-text mails, one revision per mail (with mail 0 being the summary/overview of the serie), and should have the ability to import those mails back into a repository.