magit-blame-mode: fix disabling of incompatible modes

See #1607.
magit-push-popup: switch keys for --force{,-with-lease}

We only keep `--force' for users of older versions of Git.  The new and
better argument `--force-with-lease' should be what is easier to use.
magit-log-select: bugfix
magit-merge-popup: change binding for magit-commit
Fix typos