Handle all values of branch.[name].rebase

Handle all values of branch.[name].rebase

BranchConfig treated this config property as a boolean, but git also
allows the values "preserve" and "interactive". Config property
pull.rebase also allows the same values.

Replace private enum PullCommand.PullRebaseMode by new public enum
BranchConfig.BranchRebaseMode and adapt all uses. Add a new setter to
PullCommand.

Note: PullCommand will treat "interactive" like "true", i.e., as a
non-interactive rebase. Not sure how "interactive" should be handled.
At least it won't balk on it.

Bug: 499482
Change-Id: I7309360f5662b2c2efa1bd8ea6f112c63cf064af
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>