git-changelog does not work with no previous tags

[limianwang]

An error occurs when attempting to generate a changelog from no previous tags:

$ git changelog
fatal: ambiguous argument 'HEAD -> master..': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'

This issue does not occur when previous tags exist.

In essence, it's #176.

[qw3rtman]

Perhaps if there are no tags, it could say something like "create a tag first". Thoughts, @hemanth?

[hemanth]

Yes, we need to check if tags are present and show appropriate message.