fsck: document and test commented & empty line skipList input

There is currently no comment syntax for the fsck.skipList, this isn't
really by design, and it would be nice to have support for comments.

Document that this doesn't work, and test for how this errors
out. These tests reveal a current bug, if there's invalid input the
output will emit some of the next line, and then go into uninitialized
memory. This is fixed in a subsequent change.

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

Documentation/config.txt

@@ -1712,10 +1712,13 @@ will only cause git to warn.
 fsck.skipList::
 	The path to a list of object names (i.e. one SHA-1 per
 	line) that are known to be broken in a non-fatal way and should
-	be ignored. This feature is useful when an established project
-	should be accepted despite early commits containing errors that
-	can be safely ignored such as invalid committer email addresses.
-	Note: corrupt objects cannot be skipped with this setting.
+	be ignored. Comments ('#') and empty lines are not supported, and
+	will error out.
++
+This feature is useful when an established project should be accepted
+despite early commits containing errors that can be safely ignored
+such as invalid committer email addresses.  Note: corrupt objects
+cannot be skipped with this setting.
 +
 Like `fsck.<msg-id>` this variable has corresponding
 `receive.fsck.skipList` and `fetch.fsck.skipList` variants.

t/t5504-fetch-receive-strict.sh

@@ -169,6 +169,27 @@ test_expect_success 'fsck with invalid or bogus skipList input' '
 	test_i18ngrep "Invalid SHA-1: \[core\]" err
 '
 
+test_expect_success 'fsck with invalid or bogus skipList input (comments & empty lines)' '
+	cat >SKIP.with-comment <<-EOF &&
+	# Some bad commit
+	0000000000000000000000000000000000000001
+	EOF
+	test_must_fail git -c fsck.skipList=SKIP.with-comment fsck 2>err-with-comment &&
+	test_i18ngrep "^fatal: Invalid SHA-1: # Some bad commit$" err-with-comment &&
+	cat >SKIP.with-empty-line <<-EOF &&
+	0000000000000000000000000000000000000001
+
+	0000000000000000000000000000000000000002
+	EOF
+	test_must_fail git -c fsck.skipList=SKIP.with-empty-line fsck 2>err-with-empty-line &&
+	test_i18ngrep "^fatal: Invalid SHA-1: " err-with-empty-line
+'
+
+test_expect_failure 'fsck no garbage output from comments & empty lines errors' '
+	test_line_count = 1 err-with-comment &&
+	test_line_count = 1 err-with-empty-line
+'
+
 test_expect_success 'push with receive.fsck.skipList' '
 	git push . $commit:refs/heads/bogus &&
 	rm -rf dst &&