changes: Add DumpStoryChanges

changes.DumpStoryChanges can be used to dump []*StoryChange into an
io.Writer. It is nicely formatted and everything.

Change-Id: 8a9cfee115
prompt: Add shortcut for cancellation panics

PanicCancel can be used to panic(ErrCancelled) while RecoverCancel can
be used to stop the panic from propagating. A handy shortcut for the
commands.

Change-Id: 5cfb691666
git: ListStoryCommits: Case-insensitive tag match

Make sure that ListStoryCommits uses case-insensitive matching in
GrepCommits to find relevant story commits.

Change-Id: 95d7fe7028
jira: Fix formatting issues

Change-Id: b2382eb3e0
release stage: Ensure no commits are left behind

From now on, release stage checks to make sure that all changes assigned
to the stories that are associated with the release to be stages are
actually present on the release branch.

In other words, it warns the user in case there are some changes being
left behind, i.e. they have not been cherry-picked onto the release
branch yet. The user is prompted to confirm the staging process if that
is the case.

Change-Id: b7f985358e