Merge branch 'so/cherry-pick-always-allow-m1'

Merge branch 'so/cherry-pick-always-allow-m1'

"git cherry-pick -m1" was forbidden when picking a non-merge
commit, even though there _is_ parent number 1 for such a commit.
This was done to avoid mistakes back when "cherry-pick" was about
picking a single commit, but is no longer useful with "cherry-pick"
that can pick a range of commits.  Now the "-m$num" option is
allowed when picking any commit, as long as $num names an existing
parent of the commit.

Technically this is a backward incompatible change; hopefully
nobody is relying on the error-checking behaviour.

* so/cherry-pick-always-allow-m1:
  t3506: validate '-m 1 -ff' is now accepted for non-merge commits
  t3502: validate '-m 1' argument is now accepted for non-merge commits
  cherry-pick: do not error on non-merge commits when '-m 1' is specified
  t3510: stop using '-m 1' to force failure mid-sequence of cherry-picks