Incorrect work dir for submodule fetch operations

Hi,
Since Git 2.21 the Windows version have frequently reported 

   error: cannot spawn git: Invalid argument

and still do in 2.34 and 2.35.

The reported error does not seem to have any bad effects, but the error 
report is an irritation.

At the time, this was reported to the Git for Windows team as 
<https://github.com/git-for-windows/git/issues/2126>

As it has not yet been fixed, the past couple of days I have investigated 
more closely.

The error is printed for a submodule.c function submodule_has_commits() 
which starts the command "rev-parse" for a submodule.

The reason for the error is that the function starts the command for 
submodule "foo", which is present in the checkout directory. However, the 
actual CWD dir is the git dir for the current module, not the checkout 
directory, and the change directory operation therefore fails.

The "rev-parse" made it possible for me to connect this problem with a 
different problem that I have been seeing at Linux and Mac, where the error 
message 

   fatal: exec 'rev-list': cd to 'foo' failed: No such file or directory

is frequently reported, especially when a branch in the parent module has 
new submodules defined.

AFAICT, the reason for the CWD being the git dir, not the checkout dir, is 
that  get_next_submodule() in submodule.c starts a fetch operation directly 
in the git dir, instead of the checkout directory

Further, submodule_has_commits() which is called by the fetch command via 
fetch_populated_submodules() assumes that its CWD is in the checkout 
directory.


After a bisect it seems the regression point for this was commit 
be76c2128234d94b47f7087152ee55d08bb65d88 
<https://github.com/git-for-windows/git/commit/be76c2128234d94b47f7087152ee55d08bb65d88>, 
which added the fetch in git dir command in get_next_submodule().

A testcase for reproducing the issue can be found in 
<https://github.com/git-for-windows/git/issues/2126#issuecomment-1064153093>
 (assumes default branch is "main"), which builds up a submodule set based 
on two Chromium repos (chromium src, and depot_tools)

I am uncertain what the proper fix for this issue should be, but my guess 
is that each command need to specify whether it is going to work relative 
to the git dir, or the checkout dir. It is IMO conceivable that other 
commands may have this kind of problem, especially in a submodule context, 
or that future features may encounter them.



-- 
Sincerely,
Yngve N. Pettersen
Vivaldi Technologies AS