fix: Omit other high-scoped config in fixtures

In addition to keeping fixture scripts from receiving global and
system scope Git configuration variables, as was already done, this
also omits configuration variables from high scopes similar to or
above the system scope, associated with the Git installation but
separate from the system scope.

The main and possibly only case where this happens is the "unknown"
scope associated with an Apple Git installation on macOS. This is a
file usually located under `/Library` or `/Applications`.

This is done by using `GIT_CONFIG_NOSYSTEM`, which suppresses both
the system scope and this separate "unknown" scope, instead of by
settng `GIT_CONFIG_SYSTEM` to a path like `/dev/null`. The latter
approach continues to be used to omit global scope config via
`GIT_CONFIG_GLOBAL` (as `git` recognized no `GIT_CONFIG_NOGLOBAL`).

Author: EliahKagan

tests/tools/src/lib.rs

@@ -603,7 +603,7 @@ fn configure_command<'a>(
         .env_remove("GIT_ASKPASS")
         .env_remove("SSH_ASKPASS")
         .env("MSYS", msys_for_git_bash_on_windows)
-        .env("GIT_CONFIG_SYSTEM", NULL_DEVICE)
+        .env("GIT_CONFIG_NOSYSTEM", "1")
         .env("GIT_CONFIG_GLOBAL", NULL_DEVICE)
         .env("GIT_TERMINAL_PROMPT", "false")
         .env("GIT_AUTHOR_DATE", "2000-01-01 00:00:00 +0000")