sha1_file: Show the the type and path to corrupt objects

Change the error message that's displayed when we encounter corrupt
objects to be more specific. We now print the type (loose or packed)
of corrupted objects, along with the full path to the file in
question.

Before:

    $ git cat-file blob 909ef997367880aaf2133bafa1f1a71aa28e09df
    fatal: object 909ef997367880aaf2133bafa1f1a71aa28e09df is corrupted

After:

    $ git cat-file blob 909ef997367880aaf2133bafa1f1a71aa28e09df
    fatal: loose object 909ef997367880aaf2133bafa1f1a71aa28e09df (stored in .git/objects/90/9ef997367880aaf2133bafa1f1a71aa28e09df) is corrupted

Knowing the path helps to quickly analyze what's wrong:

    $ file .git/objects/90/9ef997367880aaf2133bafa1f1a71aa28e09df
    .git/objects/90/9ef997367880aaf2133bafa1f1a71aa28e09df: empty

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

sha1_file.c

@@ -2086,6 +2086,7 @@ void *read_sha1_file_repl(const unsigned char *sha1,
 {
 	const unsigned char *repl = lookup_replace_object(sha1);
 	void *data = read_object(repl, type, size);
+	char *path;
 
 	/* die if we replaced an object with one that does not exist */
 	if (!data && repl != sha1)
@@ -2093,8 +2094,16 @@ void *read_sha1_file_repl(const unsigned char *sha1,
 		    sha1_to_hex(repl), sha1_to_hex(sha1));
 
 	/* legacy behavior is to die on corrupted objects */
-	if (!data && (has_loose_object(repl) || has_packed_and_bad(repl)))
-		die("object %s is corrupted", sha1_to_hex(repl));
+	if (!data) {
+		if (has_loose_object(repl)) {
+			path = sha1_file_name(sha1);
+			die("loose object %s (stored in %s) is corrupted", sha1_to_hex(repl), path);
+		}
+		if (has_packed_and_bad(repl)) {
+			path = sha1_pack_name(sha1);
+			die("packed object %s (stored in %s) is corrupted", sha1_to_hex(repl), path);
+		}
+	}
 
 	if (replacement)
 		*replacement = repl;