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>