These can be used to inject options in an implementation which performs a
recursive clone by executing an external command via crafted url and path
attributes such that it triggers a local executable to be run.

The library is not vulnerable as we do not rely on external executables but a
user of the library might be relying on that so we add this protection.

This matches this aspect of git's fix for CVE-2018-17456.

The header "config_file.h" has a list of inline-functions to access the
contents of a config backend without directly messing with the struct's
function pointers. While all these functions are called
"git_config_file_*", they are in fact completely backend-agnostic and
don't care whether it is a file or not. Rename all the function to
instead be backend-agnostic versions called "git_config_backend_*" and
rename the header to match.

When populating the list of submodule names, we use the submodule
configuration entry's name as the key in the map of submodule names.
This creates a hidden dependency on the liveliness of the configuration
that was used to parse the submodule, which is fragile and unexpected.

Fix the issue by duplicating the string before writing it into the
submodule name map.

Reported by Coverity, CID 1393237

Previous to dfda2f68e (submodule: remove the per-repo cache,
2015-04-27), we tried to cache our submodules per repository to avoid
having to reload it too frequently. As it created some headaches with
regards to multithreading, we removed that cache.

Previous to that removal, we had to compute what submodule status to
refresh. The mask computation was not removed, though, resulting in
confusing and actually dead code. While it seems like the mask is
currently in use in a conditional, it is not, as we unconditionally
assign to the mask previous to that condition.

Remove all mask computations to clean up stale code.

The function `load_submodule_names` was always being called with a
newly allocated string map, which was then getting filled by the
function. Move the string map allocation into `load_submodule_names`,
instead, and pass the whole map back to the caller in case no error
occurs. This change helps to avoid misuse by handing in pre-populated
maps.

When loading submodule names, we build a map of submodule paths and
their respective names. While looping over the configuration keys,
we do not check though whether a submodule path was seen already. This
leads to a memory leak in case we have multiple submodules with the same
path, as we just overwrite the old value in the map in that case.

Fix the error by verifying that the path to be added is not yet part of
the string map. Git does not allow to have multiple submodules for a
path anyway, so we now do the same and detect this duplication,
reporting it to the user.