rust-performance: introduce FastHashMap type alias for HashMap
Rust's default hashing is slow, because it is meant for preventing collision
attacks.
For all of the current Rust code, we don't care about those attacks, because
if an person with bad intentions has write access to your repo, you have other
issues.
I've chosen to use the TwoXHash crate because it was made by a reputable member
of the Rust community and has very good benchmarks.
For now it does not seem to improve performance by much for the current code,
but it's something else to not worry about when benchmarking code: in a
previous experiment with copytracing in Rust, it accounted for more than 10%
of the time of the entire script.
Differential Revision: https://phab.mercurial-scm.org/D7116
Test for changeset ba7c74081861
(update dirstate correctly for non-branchmerge updates)
$ hg init a
$ cd a
$ echo a > a
$ hg add a
$ hg commit -m a
$ cd ..
$ hg clone a b
updating to branch default
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ cd a
$ hg mv a b
$ hg commit -m move
$ echo b >> b
$ hg commit -m b
$ cd ../b
$ hg pull ../a
pulling from ../a
searching for changes
adding changesets
adding manifests
adding file changes
added 2 changesets with 2 changes to 1 files
new changesets e3c9b40284e1:772b37f1ca37
(run 'hg update' to get a working copy)
$ hg update
1 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ cd ..