tests: use "env" to run commands with temporary env-var settings

Ordinarily, we would say "VAR=VAL command" to execute a tested
command with environment variable(s) set only for that command.
This however does not work if 'command' is a shell function (most
notably 'test_must_fail'); the result of the assignment is retained
and affects later commands.

To avoid this, we used to assign and export environment variables
and run such a test in a subshell, like so:

        (
                VAR=VAL && export VAR &&
                test_must_fail git command to be tested
        )

But with "env" utility, we should be able to say:

        test_must_fail env VAR=VAL git command to be tested

which is much shorter and easier to read.

Signed-off-by: David Tran <unsignedzero@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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