[PATCH] t7610: use "file:///dev/null", not "/dev/null", fixes MinGW

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

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>
---

On Tue, Nov 15 2022, Taylor Blau wrote:

> On Wed, Nov 09, 2022 at 09:17:25PM -0500, Taylor Blau wrote:
>> On Thu, Nov 10, 2022 at 12:55:30AM +0100, Ævar Arnfjörð Bjarmason wrote:
>> > But we generally just fail some or all of 3..9 pretty fast, and don't
>> > start taking 20 minutes to run the test, when it took 10s before (or
>> > whatever).
>>
>> OK. I still think that in principle this is indistinguishable from not
>> running a setup test to completion.
>>
>> *But*: I'm willing to treat them differently since instead of
>> manifesting in an immediate failure later on in the suite, we hang for a
>> substantial period of time.
>>
>> So I'm content to merge this down, but I don't think it's worth
>> searching out for more of these in the future.
>
> Having merged this down to 'next', it looks like there is some CI
> fallout on the Windows tests, see:
>
>   https://github.com/ttaylorr/git/actions/runs/3473324797/jobs/5805324776
>
> I am not sure how I might have caught this earlier not having a Windows
> machine myself. Regardless, let's make sure that it is fixed up before
> this graduates.

Sorry again :( I think my CI was quite queued up at the time, and I
figured surely *this* won't have any porability issues, but forgot
about MinGW's sneaky /dev/null string-replacement behavior.

Windows CI passes with this:
https://github.com/avar/git/actions/runs/3473817195

 t/t7610-mergetool.sh | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/t/t7610-mergetool.sh b/t/t7610-mergetool.sh
index b1ba0d9a088..7b957022f1a 100755
--- a/t/t7610-mergetool.sh
+++ b/t/t7610-mergetool.sh
@@ -33,7 +33,7 @@ test_expect_success 'setup' '
 		git add foo &&
 		git commit -m "Add foo"
 	) &&
-	git submodule add /dev/null submod &&
+	git submodule add file:///dev/null submod &&
 	git add file1 "spaced name" file1[1-4] subdir/file3 .gitmodules submod &&
 	git commit -m "add initial versions" &&
 
@@ -614,7 +614,7 @@ test_expect_success 'submodule in subdirectory' '
 		)
 	) &&
 	test_when_finished "rm -rf subdir/subdir_module" &&
-	git submodule add /dev/null subdir/subdir_module &&
+	git submodule add file:///dev/null subdir/subdir_module &&
 	git add subdir/subdir_module &&
 	git commit -m "add submodule in subdirectory" &&
 
-- 
2.38.0.1473.g172bcc0511c