This pull request adds support for the --dry-run switch (-n) in git-svn rebase, dcommit, branch and tag commands. Branching and tagging against a svn repo are one of those somewhat infrequent operations which you really want to get right the first time.
The --dry-run switch allows us to check the magit-process buffer and catch common mistakes when dealing with these commands.
In order to reuse the existing key-mode handling for switches and magit-custom-options, all interactive commands were refactored to use magit-define-command like the rest of magit.
I'm still not done testing the code, comments and feedback welcome!
use magit-define-command for magit-svn-* commands
so that we can use switches and pass them through magit-custom-options
add 'switches key in empty key-mode groups
Modify `magit-key-mode-add-group' so that newly created groups
include the 'switches key by default. This prevents
`magit-key-mode-insert-switch' from failing when trying to add
switches to new groups created at runtime.
add --dry-run switch (-n) to magit-svn