Add a basic tutorial for the tools in depot_tools.

docs/src/demo_repo.sh

+#!/bin/bash
+
+. common_demo_functions.sh
+
+TDIR=$(mktemp --tmpdir -d demo_repo.XXXXXXXXXX)
+trap "rm -rf $TDIR" EXIT
+
+cd $TDIR
+silent git clone "$REMOTE" .
+silent git reset --hard stage_1
+silent git update-ref refs/remotes/origin/master stage_1
+silent git tag -d $(git tag -l 'stage_*')
+silent git checkout origin/master
+silent git branch -d master
+silent git config color.ui always
+
+if [[ ! "$BLANK_DEMO"  ]]
+then
+  silent git new-branch cool_feature
+
+  c "Add widget"
+  c "Refactor spleen"
+  silent git tag spleen_tag
+
+  c "another improvement"
+
+  silent git new-branch --upstream_current subfeature
+  c "slick commenting action"
+  c "integrate with CoolService"
+
+  silent git checkout cool_feature
+  c "Respond to CL comments"
+
+  silent git new-branch fixit
+  c "Epic README update"
+  c "Add neat feature"
+
+  silent git new-branch --upstream_current frozen_branch
+  c "a deleted file"
+  c "modfile"
+  c "FREEZE.unindexed"
+fi
+