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

t/t4014-format-patch.sh

@@ -1072,7 +1072,7 @@ test_expect_success '--from omits redundant in-body header' '
 '
 
 test_expect_success 'in-body headers trigger content encoding' '
-	GIT_AUTHOR_NAME="éxötìc" test_commit exotic &&
+	test_env GIT_AUTHOR_NAME="éxötìc" test_commit exotic &&
 	test_when_finished "git reset --hard HEAD^" &&
 	git format-patch -1 --stdout --from >patch &&
 	cat >expect <<-\EOF &&

t/test-lib-functions.sh

@@ -939,3 +939,25 @@ mingw_read_file_strip_cr_ () {
 		eval "$1=\$$1\$line"
 	done
 }
+
+# Like "env FOO=BAR some-program", but run inside a subshell, which means
+# it also works for shell functions (though those functions cannot impact
+# the environment outside of the test_env invocation).
+test_env () {
+	(
+		while test $# -gt 0
+		do
+			case "$1" in
+			*=*)
+				eval "${1%%=*}=\${1#*=}"
+				eval "export ${1%%=*}"
+				shift
+				;;
+			*)
+				"$@"
+				exit
+				;;
+			esac
+		done
+	)
+}