Rollup merge of #50598 - whitfin:unnecessary-mut-borrow, r=michaelwoerister

Remove unnecessary mutable borrow and resizing in DepGraph::serialize

I might be mistaken, but I noticed this whilst in this file for something else. It appears that this mutable borrow is unnecessary and since it's locking it should be removed. The resizing looks redundant since nothing additional is added to the fingerprints in this function, so that can also be removed.

diff --cc src/librustc/dep_graph/graph.rs
Simple merge