config: don't write duplicate section

We should notice that we are in the correct section to add. This is a
cosmetic bug, since replacing any of these settings does work.

This special-casing ignores that we might have a locked file, so the
hashtable does not represent the contents of the file we want to
write. This causes multivar writes to overwrite entries instead of add
to them when under lock.

There is no need for this as the normal code-path will write to the file
just fine, so simply get rid of it.

Query the `GIT_CONFIG_LEVEL_PROGRAMDATA` location when setting it up
for tests, in case the test runner has sandboxed it.

We currently use the timestamp in order to decide whether a config file
has changed since we last read it.

This scheme falls down if the file is written twice within the same
second, as we fail to detect the file change after the first read in
that second.

This is where portable git stores the global configuration which we can
use to adhere to it even though git isn't quite installed on the system.