Command

`find . -name .git -type d | xargs -n1 -P4 -I% git --git-dir=% --work-tree=%/.. remote update -p`

Using -type d prevents execution for ".git" files inside submodules. Using -P4 runs four fetches in parallel to speed things up a bit (and continues to make progress elsewhere if a single repository hangs). I prefer git remote update -p instead of git pull so that I can deal with merge conflicts later or rebase after I'm offline.