t7610: use "file:///dev/null", not "/dev/null", fixes MinGW
On MinGW the "/dev/null" is translated to "nul" on command-lines, even
though as in this case it'll never end up referring to an actual file.
So on Windows the fix for the previous "example.com" timeout issue in
8354cf752ec (t7610: fix flaky timeout issue, don't clone from
example.com, 2022-11-05) would yield:
fatal: repo URL: 'nul' must be absolute or begin with ./|../
Let's evade this yet again by prefixing this with "file://", which
makes this pass in the Windows CI.
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>