t0450: add allowlist for builtins with missing .adoc

t0450: add allowlist for builtins with missing .adoc

Before we were silently skipping all builtins that don't have a matching
.adoc file. This is overly loose and might skip documentation files
when it shouldn't, for example when there was a typo in the filename.

To ensure no new builtins are added without documentation, add an
allowlist: t0450/adoc-missing. In this file only builtin commands that
do *not* have a corresponding .adoc file shall be listed. If there is a
mismatch, fail the test. This should force future contributions to
either add an .adoc, or add the builtin name to the allowlist file.

Signed-off-by: Toon Claes <toon@iotcl.com>
[jc: squashed Patrick's "missing file fix" in]
Signed-off-by: Junio C Hamano <gitster@pobox.com>