Answer
Run the following from the parent directory, plugins in this case:
find . -type d -depth 1 -exec git --git-dir={}/.git --work-tree=$PWD/{} pull origin master \;
To clarify:
find .searches the current directory-type dto find directories, not files-depth 1for a maximum depth of one sub-directory-exec {} \;runs a custom command for every findgit --git-dir={}/.git --work-tree=$PWD/{} pullgit pulls the individual directories
To play around with find, I recommend using echo after -exec to preview, e.g.:
find . -type d -depth 1 -exec echo git --git-dir={}/.git --work-tree=$PWD/{} status \;
Note: if the -depth 1 option is not available, try -mindepth 1 -maxdepth 1.