Add stub git-drover documentation.

man/src/git-drover.demo.1.sh

+#!/usr/bin/env bash
+. demo_repo.sh
+
+silent git push origin refs/remotes/origin/master:refs/branch-heads/9999
+silent git config --add remote.origin.fetch \
+  +refs/branch-heads/*:refs/remotes/branch-heads/*
+silent git fetch origin
+
+silent git checkout -b master origin/master
+add modified_file
+set_user some.committer
+c "This change needs to go to branch 9999"
+silent git tag pick_commit
+
+comment Before working with branches, you must \'gclient sync \
+  --with_branch_heads\' at least once to fetch the branches.
+
+set_user branch.maintainer
+tick 1000
+
+run git log -n 1 --pretty=fuller
+run git checkout -b drover_9999 branch-heads/9999
+echo "# DO NOT leave off the '-x' flag"
+run git cherry-pick -x $(git show-ref -s pick_commit)
+run git log -n 1 --pretty=fuller
+pcommand git cl upload
+echo "# Get LGTM or TBR."
+run git cl land