Add a package branch name dropdown menu to the upload form.

This is useful if you want to force a new package to be added to a certain branch (i.e. replace an existing package).
For example, Adobe Reader gets named "Adobe_Reader_XI_Installer". You might have changed this to "Adobe_Reader".
When you now upload a new version, it would again be named "Adobe_Reader_XI_Installer" and you would have no way of moving it to the existing "Adobe_Reader" branch.
This patch fixes the issue by allowing you to specify "Adobe_Reader" before uploading.
If you don't specify anything, the default name as determined by makepkginfo will be used.

I have been running this patch in production for almost two months now and it works fine. I have added many packages to the automatically selected package branch and a couple using the manual branch selection from this patch.