Add 'revert' example to git-drover docs.

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

Index: man/src/git-drover.demo.common.sh
diff --git a/man/src/git-drover.demo.common.sh b/man/src/git-drover.demo.common.sh
new file mode 100755
index 0000000000000000000000000000000000000000..006e99de2bdcdfeefdbd9df710ec81365162389b
--- /dev/null
+++ b/man/src/git-drover.demo.common.sh
@@ -0,0 +1,19 @@
+#!/usr/bin/env bash
+. demo_repo.sh
+
+# Add and tag a dummy commit to refer to later.
+drover_c() {
+  add modified_file
+  set_user some.committer
+  c "$1"
+  silent git tag -f pick_commit
+  set_user branch.maintainer
+  tick 1000
+}
+
+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