git: openjdk/jtharness: (enhancement) explicitly specify --git-dir when discovering commit ID using git

[Dmitry Bessonov]

Changeset: 8a0bc399
Author:    Dmitry Bessonov <dmitry.bessonov at oracle.com>
Date:      2021-04-25 22:08:54 +0000
URL:       https://git.openjdk.java.net/jtharness/commit/8a0bc399

(enhancement) explicitly specify --git-dir when discovering commit ID using git

! build/build.xml