Merge branch 'en/pull-conflicting-options'

"git pull" had various corner cases that were not well thought out around its --rebase backend, e.g. "git pull --ff-only" did not stop but went ahead and rebased when the history on other side is not a descendant of our history. The series tries to fix them up. * en/pull-conflicting-options: pull: fix handling of multiple heads pull: update docs & code for option compatibility with rebasing pull: abort by default when fast-forwarding is not possible pull: make --rebase and --no-rebase override pull.ff=only pull: since --ff-only overrides, handle it first pull: abort if --ff-only is given and fast-forwarding is impossible t7601: add tests of interactions with multiple merge heads and config t7601: test interaction of merge/rebase/fast-forward flags and options