[GSoC][PATCH v2 09/13] submodule foreach: correct '$path' in nested submodules from a subdirectory

When running 'git submodule foreach' from a subdirectory of your repository, nested submodules get a bogus value for $sm_path: For a submodule 'sub' that contains a nested submodule 'nested', running 'git -C dir submodule foreach echo $path' would report path='../nested' for the nested submodule. The first part '../' is derived from the logic computing the relative path from $pwd to the root of the superproject. The second part is the submodule path inside the submodule. This value is of little use and is hard to document.

There are two different possible solutions that have more value:
(a) The path value is documented as the path from the toplevel of the
    superproject to the mount point of the submodule.
    In this case we would want to have path='sub/nested'.

(b) As Ramsay noticed the documented value is wrong. For the non-nested
    case the path is equal to the relative path from $pwd to the
    submodules working directory. When following this model,
    the expected value would be path='../sub/nested'.

The behavior for (b) was introduced in 091a6eb0fe (submodule: drop the top-level requirement, 2013-06-16) the intent for $path seemed to be relative to $cwd to the submodule worktree, but that did not work for nested submodules, as the intermittent submodules were not included in the path.

If we were to fix the meaning of the $path using (a) such that "path" is "the path from the toplevel of the superproject to the mount point of the submodule", we would break any existing submodule user that runs foreach from non-root of the superproject as the non-nested submodule '../sub' would change its path to 'sub'.

If we would fix the meaning of the $path using (b), such that "path" is "the relative path from $pwd to the submodule", then we would break any user that uses nested submodules (even from the root directory) as the 'nested' would become 'sub/nested'.

Both groups can be found in the wild. The author has no data if one group outweighs the other by large margin, and offending each one seems equally bad at first. However in the authors imagination it is better to go with (a) as running from a sub directory sounds like it is carried out by a human rather than by some automation task. With a human on the keyboard the feedback loop is short and the changed behavior can be adapted to quickly unlike some automation that can break silently.

Discussed-with: Ramsay Jones <ramsay@ramsayjones.plus.com>
Signed-off-by: Prathamesh Chavan <pc44800@gmail.com>
Signed-off-by: Stefan Beller <sbeller@google.com>
---
 git-submodule.sh             |  1 -
 t/t7407-submodule-foreach.sh | 36 ++++++++++++++++++++++++++++++++++--
 2 files changed, 34 insertions(+), 3 deletions(-)