changelog fails when log includes `HEAD -> master`

[tocker]

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>...]'