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>

Author: To1ne

t/t0450-txt-doc-vs-help.sh

@@ -112,10 +112,19 @@ do
 	adoc="$(builtin_to_adoc "$builtin")" &&
 	preq="$(echo BUILTIN_ADOC_$builtin | tr '[:lower:]-' '[:upper:]_')" &&
 
-	if test -f "$adoc"
+	# If and only if *.adoc is missing, builtin shall be listed in t0450/adoc-missing.
+	if grep -q "^$builtin$" "$TEST_DIRECTORY"/t0450/adoc-missing
 	then
+		test_expect_success "$builtin appropriately marked as not having .adoc" '
+			! test -f "$adoc"
+		'
+	else
 		test_set_prereq "$preq"
-	fi &&
+
+		test_expect_success "$builtin appropriately marked as having .adoc" '
+			test -f "$adoc"
+		'
+	fi
 
 	# *.adoc output assertions
 	test_expect_success "$preq" "$builtin *.adoc SYNOPSIS has dashed labels" '

t/t0450/adoc-missing

@@ -0,0 +1,9 @@
+checkout--worker
+merge-ours
+merge-recursive
+merge-recursive-ours
+merge-recursive-theirs
+merge-subtree
+pickaxe
+submodule--helper
+upload-archive--writer