[micro] Use 'env' on test_must_fail as appropriate

[Junio C Hamano <gitster@pobox.com>]

Because "VAR=VAL command" is sufficient to run 'command' with
environment variable VAR set to value VAL without affecting the
environment of the shell itself, but we cannot do the same with a
shell function (most notably, "test_must_fail"), we have subshell
invocations with multiple lines like this:

	... &&
	(
        	VAR=VAL &&
                export VAR &&
                test_must_fail git command
	) &&
        ...

which could be expressed as

	... &&
        test_must_fail env VAR=VAL git comand &&
	...

Find and shorten such constructs in existing test scripts.

Note that I am not 100% convinced myself that it is a good idea to
do this, so please do not add this to the list without seeing it
discussed.

Thanks.