Due to security concerns the commit e8d0608 (submodule: require the
submodule path to contain directories only, 2024-03-26) introduced some
checks and hard `exit(128)` calls for the case that a submodule path
contains symbolic links. This change was motivated by CVE-2024-32002,
which was discovered shortly before. Consequently, git currently aborts
actions of which some could be continued by not processing the faulty
module.

As an example, one of these actions that may be aborted is showing a
diff with diff.submodule set to `log` or `diff`: Let's assume a
repository where a submodule was moved and its old path replaced with a
symlink pointing to the new location. A git-show on an older commit from
before the move that also touches the module will now have only partial
output. If the submodule is the first file to list, there will be no
diff at all after the commit message. Other examples for actions that
are aborted after already being started are fetching and pushing with
their respective recurseSubmodules turned on.

Handle these cases more gracefully by returning an error value instead
of dying, if possible. This was done only in functions that already
supported returning an error.

With this change the mentioned git-show will display the full diff and
show a "(commits not present)" for the submodule - the same is done
currently when a submodule is simply moved. The fetch and push will
complete their action while skipping the faulty module and printing an
error message.

Signed-off-by: Julian Prein <[email protected]>