Merge branch 'ap/merge-backend-opts'

* ap/merge-backend-opts: Document that merge strategies can now take their own options Extend merge-subtree tests to test -Xsubtree=dir. Make "subtree" part more orthogonal to the rest of merge-recursive. pull: Fix parsing of -X<option> Teach git-pull to pass -X<option> to git-merge git merge -X<option> git-merge-file --ours, --theirs Conflicts: git-compat-util.h