Discard gh-pages commit history
Discard gh-pages commit history

No reason to keep the commit history for gh-pages. The docs
can be rebuilt at any commit at any time.

Keeping the history means `git log --graph --all` is cluttered
with gh-pages commits. Even worse, they appear above the
important commits because every commit to master is followed
a few minutes later by a commit to gh-pages.

sfackler/linked-hash-map,sfackler/linked-hash-map,contain-rs/linked-hash-map