git: update to v2.10.2 again

The submodule was accidentally downgraded in commit 8e9ddd21 (Bump
version, 2016-11-23).  Restore v2.10.2 so it matches the makefile again.

Signed-off-by: John Keeping <john@keeping.me.uk>

git

@@ -1 +1 @@
-Subproject commit 6406bdc0b95715a087fdeeb0f6adf3deb80a25b8
+Subproject commit ac84098b7e32406a982ac01cc76a663d5605224b