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