Add a basic tutorial for the tools in depot_tools.

docs/src/git-squash-branch.demo.1.sh

Index: docs/src/git-squash-branch.demo.1.sh
diff --git a/docs/src/git-squash-branch.demo.1.sh b/docs/src/git-squash-branch.demo.1.sh
new file mode 100755
index 0000000000000000000000000000000000000000..8d83a036d2f1586331a51ca8f489fa7d74476f78
--- /dev/null
+++ b/docs/src/git-squash-branch.demo.1.sh
@@ -0,0 +1,12 @@
+#!/bin/sh
+. demo_repo.sh
+
+run git map
+echo
+
+# since these are all empty commits, pretend there's something there
+pcommand git squash-branch -m "cool squash demo"
+git squash-branch -m "cool squash demo" > /dev/null 2> /dev/null
+c "cool squash demo"
+
+run git map