Isolate test suite git commands from ambient global/system git config

Central_test_helpers.init_repo already isolates the throwaway test repos
from the local machine's user.name/user.email, but nothing else: a global
commit.gpgsign (blocking on a hardware key or gpg-agent prompt) or a global
core.hooksPath (running pre-commit/pre-push hooks) still apply to every
commit and push the test suite makes, and can turn an otherwise ~1-2s test
run into one that hangs or crawls for reasons that have nothing to do with
central itself.

Point GIT_CONFIG_GLOBAL and GIT_CONFIG_SYSTEM at /dev/null for the test
process - git's documented way to load no config file there. The spawned
central.exe subprocess (and any git it shells out to) inherits this
process's environment, so this isolates every git invocation the suite
makes, not just the ones issued directly.

Verified with a fake HOME carrying commit.gpgsign pointed at a slow signer
plus a sleeping core.hooksPath hook: one partition (todo.ml) went from
3.28s to 0.32s (~10x) with this change, and dune build @ALL @runtest /
@fmt / @lint all still pass clean.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: mbarbin

src/test-helpers/central_test_helpers.ml

@@ -4,6 +4,23 @@
 (*  SPDX-License-Identifier: MIT                                                 *)
 (*********************************************************************************)
 
+(* Every git invocation the test suite makes - both the ones issued directly
+   from this process and the ones the spawned [central] subprocess makes on
+   its own (see [Central_test_harness.run]; it inherits this process's
+   environment) - is pointed away from the ambient global/system git config.
+   Without this, [init_repo] below only isolates identity (user.name/email);
+   anything else set globally on the machine running the tests still applies
+   to these throwaway repos, e.g. commit/tag signing (which can block on a
+   hardware key or a gpg-agent prompt) or a global [core.hooksPath] running
+   pre-commit/pre-push hooks. Either can turn an otherwise-fast test suite
+   into one that hangs or crawls, for reasons that have nothing to do with
+   [central] itself and everything to do with the developer's own [~/.gitconfig].
+   [/dev/null] is git's documented way to say "load no file here". *)
+let () =
+  Unix.putenv "GIT_CONFIG_GLOBAL" "/dev/null";
+  Unix.putenv "GIT_CONFIG_SYSTEM" "/dev/null"
+;;
+
 module Fake_subrepo = struct
   type t =
     { subrepo : Central.Subrepo.t