"git submodule" vs "git subtree" vs "repo" vs "git subdir" ... ?

["Robert P. J. Day" <rpjday@crashcourse.ca>]

  looking for general opinions ... i am (frighteningly :-) teaching a
git course later this week, and one of the topics on the list is git
submodules, which was specifically requested by the client as their
idea of how to start incorporating child repos in new projects.

  however, given the number of articles written about the drawbacks
with submodules, i wanted to throw in a section about "git subtree" as
well, even though (as discussed earlier) there is some minor dispute
as to whether "git subtree" is part of "core" git, but i'm not going
to let that stop me.

  going even beyond that, there is also google's "repo" command, which
i seem to see more and more often, like here for automotive grade
linux:

  https://wiki.automotivelinux.org/agl-distro/source-code

and it would be a shame to at least not mention that as yet another
possibility.

  and then there are unofficial, hand-rolled solutions, like
"git-subdir":

  https://github.com/andreyvit/git-subdir

given that the client does not appear to be wedded to any particular
solution yet, i'm open to recommendations or pointers to online
comparisons that i'll collect and post on a single wiki page, and they
can peruse the comparisons at their leisure.

  so ... thoughts? no need to be verbose, just links to decent online
discussions/comparisons would be massively useful.

rday

p.s. oh, pointers to well-designed usage of any of the above would be
handy as well. as i mentioned, for "repo", there's AGL. for
submodules, i might use boost:

  https://github.com/boostorg/boost/wiki/Getting-Started

and so on. thank you kindly.