Move to the `git_error` name in the internal API for error-related
functions.

When we try to add a configuration file with `git_config_add_file_ondisk`, we
treat nonexisting files as empty. We do this by performing a stat call, ignoring
ENOENT errors. This works just fine in case the file or any of its parents
simply does not exist, but there is also the case where any of the parent
directories is not a directory, but a file. So e.g. trying to add a
configuration file "/dev/null/.gitconfig" will fail, as `errno` will be ENOTDIR
instead of ENOENT.

Catch ENOTDIR in addition to ENOENT to fix the issue. Add a test that verifies
we are able to add configuration files with such an invalid path file just fine.

config: Port config_file_fuzzer to the new in-memory backend.

In case a configuration includes a key "include.path=" without any
value, the generated configuration entry will have its value set to
`NULL`. This is unexpected by the logic handling includes, and as soon
as we try to calculate the included path we will unconditionally
dereference that `NULL` pointer and thus segfault.

Fix the issue by returning early in both `parse_include` and
`parse_conditional_include` in case where the `file` argument is `NULL`.
Add a test to avoid future regression.

The issue has been found by the oss-fuzz project, issue 10810.

While our tests in config::include create a plethora of configuration
files, most of them do not get removed at the end of each test. This can
cause weird interactions with tests that are being run at a later stage
if these later tests try to create files or directories with the same
name as any of the created configuration files.

Fix the issue by unlinking all created files at the end of these tests.

Now that we have abstracted away how to store and retrieve config
entries, it became trivial to implement a new in-memory backend by
making use of this. And thus we do so.

This commit implements a new read-only in-memory backend that can parse
a chunk of memory into a `git_config_backend` structure.

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.