use
git log --author=<name> --no-merges
Additionally the --first-parent option may give useful result for you:
–first-parent
Follow only the first parent commit upon seeing a merge commit. This option can give a better overview when viewing the
evolution of a particular topic branch, because merges
into a topic branch tend to be only about adjusting to updated upstream from time to time, and this option allows you to
ignore the individual commits brought in to your
history by such a merge. Cannot be combined with –bisect.