test-lib: add in-shell "env" replacement

The one-shot environment variable syntax:

  FOO=BAR some-program

is unportable when some-program is actually a shell
function, like test_must_fail (on some shells FOO remains
set after the function returns, and on others it does not).

We sometimes get around this by using env, like:

  test_must_fail env FOO=BAR some-program

But that only works because test_must_fail's arguments are
themselves a command which can be run. You can't run:

  env FOO=BAR test_must_fail some-program

because env does not know about our shell functions. So
there is no equivalent for test_commit, for example, and one
must resort to:

  (
    FOO=BAR
    export FOO
    test_commit
  )

which is a bit verbose.  Let's add a version of "env" that
works _inside_ the shell, by creating a subshell, exporting
variables from its argument list, and running the command.

Its use is demonstrated on a currently-unportable case in
t4014.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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