submodule test: Use config.unset instead of setting to null

submodule test: Use config.unset instead of setting to null

Most relative-URL tests for SubmoduleInitCommand carry out the following
steps:

 1. add a submodule at path "sub" to the index
 2. set remote.origin.url in .git/config
 3. configure .gitmodules, possibly using relative URLs, and see what
    happens

resolveWorkingDirectoryRelativeUrl() is meant to test the fallback when
remote.origin.url is not set, to match C git which treats the URL as
relative to the cwd in that case.  To do so, in step (2) it sets
remote.origin.url to null.

However, Config.setString when taking a null value does not actually
unset that value from the configuration --- it sets it to the empty
string.  This means we are testing a behavior that C git never
supported.  Use Config.unset instead.

Change-Id: I7af29fbbd333a2598843d62c320093c48b2ad972
Signed-off-by: Jonathan Nieder <jrn@google.com>