test-lib-functions: test_cmp: eval $GIT_TEST_CMP

Shell recognises first non-assignment token as command name.
With /bin/sh linked to either /bin/bash or /bin/dash,
`cd t/perf && ./p0000-perf-lib-sanity.sh -d -i -v` reports:

> test_cmp:1: command not found: diff -u

Using `eval` to unquote $GIT_TEST_CMP as same as precedence in `git_editor`.

Signed-off-by: Đoàn Trần Công Danh <congdanhqx@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/test-lib-functions.sh

@@ -905,7 +905,7 @@ test_expect_code () {
 # - not all diff versions understand "-u"
 
 test_cmp() {
-	$GIT_TEST_CMP "$@"
+	eval "$GIT_TEST_CMP" '"$@"'
 }
 
 # Check that the given config key has the expected value.