pull: accept --rebase=merges to recreate the branch topology

pull: accept --rebase=merges to recreate the branch topology

Similar to the `preserve` mode simply passing the `--preserve-merges`
option to the `rebase` command, the `merges` mode simply passes the
`--rebase-merges` option.

This will allow users to conveniently rebase non-trivial commit
topologies when pulling new commits, without flattening them.

Signed-off-by: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>