Merge branch 'da/subtree-2.9-regression' into maint

Merge branch 'da/subtree-2.9-regression' into maint

"git merge" in Git v2.9 was taught to forbid merging an unrelated
lines of history by default, but that is exactly the kind of thing
the "--rejoin" mode of "git subtree" (in contrib/) wants to do.
"git subtree" has been taught to use the "--allow-unrelated-histories"
option to override the default.

* da/subtree-2.9-regression:
  subtree: fix "git subtree split --rejoin"
  t7900-subtree.sh: fix quoting and broken && chains