fsck: optionally ignore specific fsck issues completely

An fsck issue in a legacy repository might be so common that one would
like not to bother the user with mentioning it at all. With this change,
that is possible by setting the respective message type to "ignore".

This change "abuses" the missingEmail=warn test to verify that "ignore"
is also accepted and works correctly. And while at it, it makes sure
that multiple options work, too (they are passed to unpack-objects or
index-pack as a comma-separated list via the --strict=... command-line
option).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

fsck.h

@@ -3,6 +3,7 @@
 
 #define FSCK_ERROR 1
 #define FSCK_WARN 2
+#define FSCK_IGNORE 3
 
 struct fsck_options;