Draft: Add --incremental, --default-branch, --rename options to fast-export

My workflow requires me to import the history of single files into an existing git repository. I added some options to fast-export to facilitate this. I'm not sure if you would be interested in adding any of these features, so I haven't bothered making a test case yet.

• --incremental restarts an incremental from the default branch head. This could be changed to require an argument specifying which branch to use.
• --default-branch sets the name of the branch to use instead of trunk since it isn't always master.
• --rename renames the file being exported. This is useful to change where the file is placed in the git directory.