[RFC PATCHv2 00/17] Checkout aware of Submodules!

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

Stefan Beller (17):
  submodule.h: add extern keyword to functions
  submodule: modernize ok_to_remove_submodule to use argv_array
  update submodules: move up prepare_submodule_repo_env
  update submodules: add is_submodule_populated
  update submodules: add submodule config parsing
  update submodules: add a config option to determine if submodules are
    updated
  update submodules: introduce submodule_is_interesting
  update submodules: add depopulate_submodule
  update submodules: add scheduling to update submodules
  update submodules: is_submodule_checkout_safe
  unpack-trees: teach verify_clean_submodule to inspect submodules
  unpack-trees: remove submodule contents if interesting
  entry: write_entry to write populate submodules
  submodule: teach unpack_trees() to update submodules
  checkout: recurse into submodules if asked to
  completion: add '--recurse-submodules' to checkout
  checkout: add config option to recurse into submodules by default

 Documentation/config.txt               |   6 +
 Documentation/git-checkout.txt         |   8 +
 builtin/checkout.c                     |  31 +++-
 cache.h                                |   2 +
 contrib/completion/git-completion.bash |   2 +-
 entry.c                                |  14 +-
 submodule-config.c                     |  22 +++
 submodule-config.h                     |  17 +-
 submodule.c                            | 292 +++++++++++++++++++++++++++++---
 submodule.h                            |  74 ++++++---
 t/t2013-checkout-submodule.sh          | 293 ++++++++++++++++++++++++++++++++-
 t/t9902-completion.sh                  |   1 +
 unpack-trees.c                         |  92 ++++++++---
 unpack-trees.h                         |   1 +
 14 files changed, 768 insertions(+), 87 deletions(-)