Use 'git merge -Xsubtree' when git version >= 1.7.0.

Use 'git merge -Xsubtree' when git version >= 1.7.0.

It's possible to specify the subdir of a subtree since Git 1.7.0 - adding
support for that functionality to make the merge more stable.

Also checking for git version - now only uses the new subtree subdir option
when on at least 1.7.