Catch "HEAD -> master" when creating the changelog

[tocker]

Fixes #395.

git log may report (HEAD -> master, tag:...) instead of (HEAD, tag:...).

3582b9a$        2015-07-19$      (HEAD -> master, tag: v0.6, tag: lightweight)

This causes changelog to fail:

$ git changelog -x -p
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 PR will fix this issue by matching anything coming after HEAD up to the first comma.

[hemanth]

Thanks 👍

[tocker]

That was fast! 😄

[hemanth]

😉