test: fix tests use of 'file' protocol

In response to CVE-2022-39253, Git now considers the `file://` protocol
to be unsafe by default. The default value of the  `protocol.file.allow`
config variable was changed to `user` [1], meaning that a file URL or a
local path is only trusted if it came directly from user input, and not
if it came through a command which executes a clone/fetch/push
internally.

The tests fall foul of this new requirement by attempting to run a
`git submodule add` with a local directory. Internally, this performs a
clone, which is no longer trusted because of the change described above.
This results in the command failing with a "transport 'file' not
allowed" message.

Since this is only the case for a single command, then fix the test by
setting `protocol.file.allow` to `always` when we run it.

 [1] https://github.blog/2022-10-18-git-security-vulnerabilities-announced/#cve-2022-39253

Author: owbone

git_sizer_test.go

@@ -701,7 +701,7 @@ func TestSubmodule(t *testing.T) {
 	require.NoError(t, cmd.Run(), "creating main commit")
 
 	// Make subm a submodule of main:
-	cmd = mainRepo.GitCommand(t, "submodule", "add", submRepo.Path, "sub")
+	cmd = mainRepo.GitCommand(t, "-c", "protocol.file.allow=always", "submodule", "add", submRepo.Path, "sub")
 	cmd.Dir = mainRepo.Path
 	require.NoError(t, cmd.Run(), "adding submodule")