Add a basic tutorial for the tools in depot_tools.

docs/src/common_demo_functions.sh

+#!/bin/bash
+
+REMOTE=$(pwd)/demo_repo
+
+unset GIT_DIR
+
+# Helper functions
+set_user() {
+  export GIT_AUTHOR_EMAIL="$1@chromium.org"
+  export GIT_AUTHOR_NAME="$1"
+  export GIT_COMMITTER_EMAIL="$1@chromium.org"
+  export GIT_COMMITTER_NAME="$1"
+}
+set_user 'local'
+
+
+# increment time by X seconds
+TIME=1397119976
+tick() {
+  TIME=$[$TIME + $1]
+  export GIT_COMMITTER_DATE="$TIME +0000"
+  export GIT_AUTHOR_DATE="$TIME +0000"
+}
+tick 0
+
+# a commit
+c() {
+  silent git commit --allow-empty -m "$1"
+  tick 10
+}
+
+praw() {
+  echo -e "\x1B[37;1m$ $@\x1B[m"
+}
+
+# print a visible command (but don't run it)
+pcommand() {
+  praw "$(python -c '\
+    import sys, pipes; \
+    print " ".join(map(pipes.quote, sys.argv[1:]))' "$@")"
+}
+
+# run a visible command
+run() {
+  pcommand "$@"
+  "$@"
+}
+
+comment() {
+  echo "# $@"
+}
+
+# run a silent command
+silent() {
+  if [[ $DEBUG ]]
+  then
+    "$@"
+  else
+    "$@" > /dev/null 2> /dev/null
+  fi
+}
+
+# add a file with optionally content
+add() {
+  local CONTENT=$2
+  if [[ ! $CONTENT ]]
+  then
+    CONTENT=$(python -c 'import random, string; \
+      print "".join(random.sample(string.lowercase, 16))')
+  fi
+  echo "$CONTENT" > $1
+  silent git add $1
+}
+
+# Add a special callout marker at the given line offset to indicate to
+# filter_demo_output.py to add a callout at that offset.
+callout() {
+  echo -e "\x1b[${1}c"
+}