* Only update _fast-forwardable_ branches to their upstreams. This uses
`git merge --no-ff` for the active branch and `git branch --force`
for others. Checks for clean working directories and ancestry are
performed as expected.
* Added --fetch-only / -f to disable branch updating entirely, if
desired.
* Deprecated --merge and --rebase. These have no effect now and instead
give a warning.