Allow to rebase+merge with --no-ff

[jd]

Feature requested from isaacs/github#1135 and isaacs/github#1143 and isaacs/github#1017

[ljharb]

Also, allow for rebase+merge with --ff-only.

[garyverhaegen-da]

Can't you do that with the existing actions? From my understanding of the documentation,

- method: merge
  strict: true
  strict_method: rebase

should give you the semi-linear history, at least if method: merge is equivalent to pressing the merge button on GitHub (which includes --no-ff), and

- method: rebase
  strict: true
  strict_method: rebase

should give you the linear version (i.e. --ff-only) if it matches pressing the "rebase" button on GitHub.