Fix annoying warnings when running in a container

[troglobit]

This is a follow-up to 07f28a4 which added support for figuring out the GIT revision when 9pm is a submodule in another proejct.

However, that project in turn may be a git worktree, where the actual .git directory is elsewhere, and when said project is running its tests from a Docker container, things get worse. It is very likely that the actual .git directory is unreachable from within the container.

This patch hides the following annoying warnings:

warning, git command failed (Command '['git', '--git-dir', '/home/jocke/src/x-misc/test/9pm/../../../infix/.git/worktrees/x-misc/modules/9pm', 'rev-parse', 'HEAD']' returned non-zero exit status 128.)
9PM - Simplicity is the ultimate sophistication
warning, git command failed (Command '['git', '--git-dir', '/home/jocke/src/infix/.git/worktrees/x-misc', 'rev-parse', 'HEAD']' returned non-zero exit status 128.)

Testing Infix

Running suite 0001 all.yaml
    .
    . [snip]
    .
    `-- o 0031 Layer-2/3 tunnel tests
        |-- o 0032 Basic tunnel connectivity test
        |   |-- o 0033 GRE point-to-point
        |   |-- o 0034 GRETAP point-to-point
        |   `-- o 0035 VXLAN point-to-point
        `-- o 0036 Tunnel interface bridged with physical
            |-- o 0037 GRETAP bridged with physical interface
            `-- o 0038 VXLAN bridged with physical interface
warning, git command failed (Command '['git', '--git-dir', '/home/jocke/src/infix/.git/worktrees/x-misc', 'describe', '--tags', '--always']' returned non-zero exit status 128.)
warning, git command failed (Command '['git', '--git-dir', '/home/jocke/src/infix/.git/worktrees/x-misc', 'describe', '--tags', '--always']' returned non-zero exit status 128.)
warning, git command failed (Command '['git', '--git-dir', '/home/jocke/src/infix/.git/worktrees/x-misc', 'rev-parse', 'HEAD']' returned non-zero exit status 128.)

[rical]

Great, thanks!