Get username for lightweight tag from git, not hg

[dusty-phillips]

In #58, PaulPrice fixed lightweight tags such that the username is set to the default mercurial username on the host system, rather than None. This is a much improved interface.

However, I now think it would make more sense to set the username to the default git username and e-mail when pushing to mercurial. At the time that gitifyhg is exporting to mercurial, it knows (or ought to know) more about the state of the git repo than the mercurial one.

Mercurial has no concept of lightweight tags; it is therefore probably more useful for gitifyhg users to use only heavyweight tags. However, there is no reason to explicitly prohibit lightweight tags; we just have to do the heavyweightifying automatically.

[PaulPrice]

This is a good idea. Another reason is that since the user is using gitifyhg, he may not have Mercurial set up properly with the author name.

[felipec]

I've fixed this.

https://github.com/felipec/gitifyhg/commit/a2912f49a6f41a9ff5f86ba5ee484a9a0b9ebf6a