Merge branch 'ap/merge-backend-opts'

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