t: avoid git config syntax from newer releases

t: avoid git config syntax from newer releases

In a recent security release, 05e9cd64ee (config: quote values
containing CR character, 2025-05-19) added calls to `git config get`,
`git config set`, and `git config unset` which are not present on the
maint-2.43 branch.

These subcommands were added in the following commits, released in
git-2.46.0:

  4e51389000 (builtin/config: introduce "get" subcommand, 2024-05-06),
  00bbdde141 (builtin/config: introduce "set" subcommand, 2024-05-06),
  95ea69c67b (builtin/config: introduce "unset" subcommand, 2024-05-06)

Revert to the previous `git config` syntax for older maintenance
branches.

Signed-off-by: Todd Zullinger <tmz@pobox.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>