The git log command has a --perl-regexp flag [0], which allows you to filter commits that match a Perl-style regular expression. I guess in that situation you could specify a regular expression that excludes dependabot in --author.
If typing --perl-regexp gets old, you can always alias it or add a repo/global config option via grep.patternType [1]. Or you can create an alias for adding the less-powerful --invert-grep [2].