Add default option to return author date in `gert::git_log`

[jabenninghoff]

gert::git_log() returns the commit date of the commit, not the author date of the commit.

This differs from git log which shows the author date by default. (git log --pretty=fuller shows both the author date and commit date)

I expected gert::git_log() to use the author date, as does git log and other tools like GitHub Desktop.

Could this be fixed, or added as a default option for gert::git_log()?