env--helper: new undocumented builtin wrapping git_env_*()

env--helper: new undocumented builtin wrapping git_env_*()

We have many GIT_TEST_* variables that accept a <boolean> because
they're implemented in C, and then some that take <non-empty?> because
they're implemented at least partially in shellscript.

Add a helper that wraps git_env_bool() and git_env_ulong() as the
first step in fixing this. This isn't being added as a test-tool mode
because some of these are used outside the test suite.

Part of what this tool does can be done via a trick with "git config"
added in 83d842dc8c ("tests: turn on network daemon tests by default",
2014-02-10) for test_tristate(), i.e.:

    git -c magic.variable="$1" config --bool magic.variable 2>/dev/null

But as subsequent changes will show being able to pass along the
default value makes all the difference, and we'll be able to replace
test_tristate() itself with that.

The --type=bool option will be used by subsequent patches, but not
--type=ulong. I figured it was easy enough to add it & test for it so
I left it in so we'd have wrappers for both git_env_*() functions, and
to have a template to make it obvious how we'd add --type=int etc. if
it's needed in the future.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>