Meta/Reintegrate: pass custom merge options

While rebuilding an integration branch, some topics may need custom
merge options, such as -Xsubtree=where/, to explicitly tell where
that foreign history sits in our tree.  Teach the Reintegrate script
to pay attention to branch.$name.rebuild configuration, e.g.

	[branch "jl/git-gui-show-added-submodule-changes"]
		rebuild = -Xsubtree=git-gui/

to allow customization.
What's cooking (2014/04 #05)