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.

Author: dtolnay

deploy-docs.sh

@@ -10,11 +10,9 @@ git init
 git config user.email '[email protected]'
 git config user.name 'FlashCat'
 git remote add upstream "https://${GH_TOKEN}@github.com/${TRAVIS_REPO_SLUG}.git"
-git fetch upstream gh-pages
-git reset upstream/gh-pages
 
 touch .
 
 git add -A .
-git commit -m "rebuild pages at ${rev}"
-git push -q upstream HEAD:gh-pages
+git commit -qm "rebuild pages at ${rev}"
+git push -q upstream HEAD:gh-pages --force