Looping Through Subdirectories and Running a Command in Each

Problem

I have a set of repositories sorted into directories based on their VCS (Git, Mercurial, SVN). With Subversion I was able to run in the parent directory and it would loop through each directory and update each repository as expected. That's not the case for Git or Mercurial. I would like to come up with a bash script that I can run to do exactly that, loop through directories and either or in each. I just don't have much bash scripting experience.