fsck: document that skipList input must be unabbreviated

Abbreviating the SHA-1s in the skipList input has never worked, but
the documentation hasn't unambiguously stated that this is an error,
and there was no test for it.

Let's fix both since it would be easy for some later refactoring
e.g. switch to accidentally switch to a looser OID parsing function,
causing the tests before this change to pass, but for older versions
of git to be incompatible with the new skipList format.

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

Documentation/config.txt

@@ -1710,7 +1710,7 @@ doing the same for `receive.fsck.<msg-id>` and `fetch.fsck.<msg-id>`
 will only cause git to warn.
 
 fsck.skipList::
-	The path to a list of object names (i.e. one SHA-1 per
+	The path to a list of object names (i.e. one unabbreviated SHA-1 per
 	line) that are known to be broken in a non-fatal way and should
 	be ignored. Comments ('#') and empty lines are not supported, and
 	will error out.

t/t5504-fetch-receive-strict.sh

@@ -190,6 +190,12 @@ test_expect_failure 'fsck no garbage output from comments & empty lines errors'
 	test_line_count = 1 err-with-empty-line
 '
 
+test_expect_success 'fsck with invalid abbreviated skipList input' '
+	echo $commit | test_copy_bytes 20 >SKIP.abbreviated &&
+	test_must_fail git -c fsck.skipList=SKIP.abbreviated fsck 2>err-abbreviated &&
+	test_i18ngrep "^fatal: Invalid SHA-1: " err-abbreviated
+'
+
 test_expect_success 'push with receive.fsck.skipList' '
 	git push . $commit:refs/heads/bogus &&
 	rm -rf dst &&