Merge branch 'jk/shortlog-tolerate-broken-commit' into maint

"git shortlog" used to choke and die when there is a malformed commit
(e.g. missing authors); it now simply ignore such a commit and keeps
going.

* jk/shortlog-tolerate-broken-commit:
shortlog: ignore commits with missing authors

diff --cc builtin/shortlog.c
Simple merge