A very experimental alternative to git-subtree
Installation: drop the contents of this repository into your .git/hooks directory.
Usage: In the root of your repository create a file .gitsubrepos containing the names of subrepo directories, each on its own line. Commit a change in one or more of the subrepo directories and see what happens...
See http://www.mos6581.org/git_subtree_alternative for details.