Auto merge of rust-lang#8901 - Jarcho:sharing_code, r=dswij

Rework `branches_sharing_code`

fixes rust-lang#7378

This changes the lint from checking pairs of blocks, to checking all the blocks at the same time. As such there's almost none of the original code left.

changelog: Don't lint `branches_sharing_code` when using different binding names

Author: bors