[RFCv8 PATCH 00/17] Checkout aware of Submodules!

previous work: https://public-inbox.org/git/20170306205919.9713-1-sbeller@google.com/

v8:
 * Thanks Ramsay; another variable is static now!
 * Thanks Junio; fixed typos (in comments and commit messages)!
 * dropped the patch touching read-cache.c; I realized it is bogus after
   inspecting it and trying to write a commit message.
 * All commits have commit messages now.
 * diff to Junios version below.
 
v7:
 * addressed Erics comment by fixing the bashism in t/lib-submodule-update.sh
diff to v6:
  diff --git a/t/lib-submodule-update.sh b/t/lib-submodule-update.sh
  index 5caae06bc5..949ebd546c 100755
  --- a/t/lib-submodule-update.sh
  +++ b/t/lib-submodule-update.sh
  @@ -255,7 +255,7 @@ test_superproject_content () {
   # Test that the given submodule at path "$1" contains the content according
   # to the submodule commit recorded in the superproject's commit "$2"
   test_submodule_content () {
  -       if test "$1" == "-C"
  +       if test x"$1" = "x-C"
          then
                  cd "$2"
                  shift; shift;
 * interdiff to v5 below
                  

v6:
 * added support for read-tree (see last patch) to see how generic the
   code of the previous patches is. I am pretty pleased with that patch.
 * marked two functions static. Thanks Ramsay!
 * fixed the recursive test; it still fails but it is the code that fails,
   not the test. For this I had to change the setup code slightly.
 * 2 new patches adding tiny refactoring to the submodule test lib.  
 * interdiff (to origin/sb/checkout-recurse-submodules, which is v5) below.

v5:
 * as v4 was the first version queued by Junio, we do have an interdiff below!
 * renamed functions
 * changed the API, now the caller has to take care of the submodule strategy
   themselves. (Note this can be different for different situations, e.g.
   when a submodule is deleted, we can do that for any strategy except none and
   !command. But for moving to a new state of the submodule we currently
   only implement "checkout" [unspecified defaults to checkout]. warning about
   the others, doing nothing there.)

v4:
 * addressed all comments of Brian, Junio and Brandon.
 Thanks!
 * one major point of change is the introduction of another patch
   "lib-submodule-update.sh: do not use ./. as submodule remote",
   as that took some time to track down the existing bug.
 
v3:
 * moved tests from t2013 to the generic submodule library.
 * factored out the refactoring patches to be up front
 * As I redid the complete implementation, I have the impression this time
   it is cleaner than previous versions.
 
 I think we still have to fix the corner cases of directory/file/submodule 
 conflicts before merging, but this serves as a status update on my current
 way of thinking how to implement the worktree commands being aware of
 submodules.
 
Thanks,
Stefan

v2:
* based on top of the series sent out an hour ago
  "[PATCHv4 0/5] submodule embedgitdirs"
* Try to embed a submodule if we need to remove it.
* Strictly do not change behavior if not giving the new flag.
* I think I missed some review comments from v1, but I'd like to get
  the current state out over the weekend, as a lot has changed so far.
  On Monday I'll go through the previous discussion with a comb to see
  if I missed something.
  
v1:
When working with submodules, nearly anytime after checking out
a different state of the projects, that has submodules changed
you'd run "git submodule update" with a current version of Git.

There are two problems with this approach:

* The "submodule update" command is dangerous as it
  doesn't check for work that may be lost in the submodule
  (e.g. a dangling commit).
* you may forget to run the command as checkout is supposed
  to do all the work for you.

Integrate updating the submodules into git checkout, with the same safety promises that git-checkout has, i.e. not throw away data unless asked to. This is done by first checking if the submodule is at the same sha1 as it is recorded in the superproject. If there are changes we stop proceeding the checkout just like it is when checking out a file that has local changes.

The integration happens in the code that is also used in other commands such that it will be easier in the future to make other commands aware of submodule.

This also solves d/f conflicts in case you replace a file/directory with a submodule or vice versa.

The patches are still a bit rough, but the overall series seems promising enough to me that I want to put it out here.

Any review, specifically on the design level welcome!

Thanks,
Stefan