Add a basic tutorial for the tools in depot_tools.

docs/src/depot_tools_tutorial.demo.tldr.sh

+#!/bin/bash
+BLANK_DEMO=1
+. demo_repo.sh
+
+trunc() {
+  echo ... truncated output ...
+}
+
+trunc_command() {
+  pcommand "$@"
+  trunc
+}
+
+WS=build/whitespace_file.txt
+add_ws() {
+  praw cat '>>' $WS '<<EOF'
+  echo -e "$1"
+  echo EOF
+  echo -e "$1" >> $WS
+}
+
+ed_ws() {
+  echo -ne "\x1B[37;1m$ echo -e "
+  echo -n "'$1'"
+  echo -e " | ed $WS\x1B[m"
+  echo -e "$1" | ed $WS
+}
+
+# needs an extra echo afterwards
+map() {
+  run git map
+  echo
+}
+
+ED1='/Banana\ns/Banana/Kuun\nwq'
+
+ADD1="
+\"You recall what happened on Mulholland drive?\" The ceiling fan rotated slowly
+overhead, barely disturbing the thick cigarette smoke. No doubt was left about
+when the fan was last cleaned."
+
+ED2='/Kuun\ns/Kuun/Kun\nwq'
+
+ADD2="
+There was an poignant pause."
+
+ADD3="
+CHAPTER 3:
+Mr. Usagi felt that something wasn't right. Shortly after the Domo-Kun left he
+began feeling sick."
+
+trunc_command fetch chromium
+pcommand cd src
+
+comment "(only on linux)"
+trunc_command ./build/install-build-deps.sh
+
+comment "Pull in all dependencies for HEAD"
+trunc_command gclient sync
+
+comment "Let's fix something!"
+run git new-branch fix_typo
+ed_ws "$ED1"
+run git commit -am 'Fix terrible typo.'
+map
+run git status
+trunc_command git cl upload -r domo@chromium.org --send-mail
+
+comment "While we wait for feedback, let's do something else."
+run git new-branch chap2
+run git map-branches
+add_ws "$ADD1"
+run git status
+
+comment "Someone on the code review pointed out that our typo-fix has a typo :("
+comment "We're still working on 'chap2' but we really want to land"
+comment "'fix_typo', so let's switch over and fix it."
+run git freeze
+run git checkout fix_typo  2>&1
+ed_ws "$ED2"
+run git upstream-diff --wordwise
+run git commit -am 'Fix typo for good!'
+trunc_command git cl upload
+
+comment "Since we got lgtm, let the CQ land it."
+pcommand git cl set_commit
+map
+
+comment "Switch back to where we were using the nav* commands (for fun..."
+comment "git checkout would work here too)"
+run git map-branches
+run git nav-upstream  2>&1
+pcommand git nav-downstream
+git nav-downstream --pick 0  2>&1
+run git map-branches
+
+comment "Now we can pick up on chapter2 where we left off."
+run git thaw
+run git diff
+add_ws "$ADD2"
+run git diff
+run git commit -am 'Finish chapter 2'
+map
+trunc_command git cl upload -r domo@chromium.org --send-mail
+
+comment "We poke a committer until they lgtm :)"
+pcommand git cl set_commit
+
+comment "While that runs through the CQ, let's get started on chapter 3."
+comment "Since we know that chapter 3 depends on chapter 2, we'll track the"
+comment "current chapter2 branch."
+run git new-branch --upstream_current chap3
+add_ws "$ADD3"
+run git commit -am 'beginning of chapter 3'
+map
+
+comment "We haven't updated the code in a while, so let's do that now."
+pcommand git rebase-update
+echo Fetching origin
+git fetch origin 2>&1 | grep -v 'stage'
+silent git update-ref refs/remotes/origin/master stage_2
+silent git tag -d $(git tag -l 'stage_*')
+git rebase-update --no_fetch
+
+comment "Well look at that. The CQ landed our typo and chapter2 branches "
+comment "already and git rebase-update cleaned them up for us."
+trunc_command gclient sync
+map
+
+comment "Someone on IRC mentions that they actually landed a chapter 3 already!"
+comment "We should pull their changes before continuing. Brace for"
+comment "a code conflict!"
+pcommand git rebase-update
+echo Fetching origin
+git fetch origin 2>&1 | grep -v 'stage'
+silent git tag -d $(git tag -l 'stage_*')
+echo Rebasing: chap2
+silent git rebase-update
+echo ... lots of output, it\'s a conflict alright :\(...
+run git diff
+
+comment "Oh, well, that's not too bad. In fact... that's a terrible chapter 3!"
+praw \$EDITOR "$WS"
+echo "... /me deletes bad chapter 3 ..."
+silent git checkout --theirs -- "$WS"
+run git add "$WS"
+run git diff --cached
+
+comment "Much better"
+run git rebase --continue
+run git rebase-update
+silent git tag -d $(git tag -l 'stage_*')
+trunc_command gclient sync
+map
+trunc_command git cl upload