pq: pass '--no-merges' to git log

[andrewlukoshko]

When we merge changes to patch-queue/master from another repo then gbp pq export generates TWO patches: First patch from original commit, it's OK Second patch from merge commit, and it reverts previous, so nothing will change when we build a package.

Passing --no-merges to git log when we do gbp pq export fixes the problem.

[agx]

@andrewlukoshko the pq is not meant to receive branches but I'm okay with adding this if you can describe a proper use case with a unittest.

(sorry for the delay)