apply: verify submodule commit object name better

A textual patch also records the submodule commit object name in
full.  Make the parsing more robust by reading from there and
verifying the (possibly abbreviated) name on the index line matches.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/apply.c

@@ -3587,6 +3587,40 @@ static int get_current_sha1(const char *path, unsigned char *sha1)
 	return 0;
 }
 
+static int preimage_sha1_in_gitlink_patch(struct patch *p, unsigned char sha1[20])
+{
+	/*
+	 * A usable gitlink patch has only one fragment (hunk) that looks like:
+	 * @@ -1 +1 @@
+	 * -Subproject commit <old sha1>
+	 * +Subproject commit <new sha1>
+	 * or
+	 * @@ -1 +0,0 @@
+	 * -Subproject commit <old sha1>
+	 * for a removal patch.
+	 */
+	struct fragment *hunk = p->fragments;
+	static const char heading[] = "-Subproject commit ";
+	char *preimage;
+
+	if (/* does the patch have only one hunk? */
+	    hunk && !hunk->next &&
+	    /* is its preimage one line? */
+	    hunk->oldpos == 1 && hunk->oldlines == 1 &&
+	    /* does preimage begin with the heading? */
+	    (preimage = memchr(hunk->patch, '\n', hunk->size)) != NULL &&
+	    !prefixcmp(++preimage, heading) &&
+	    /* does it record full SHA-1? */
+	    !get_sha1_hex(preimage + sizeof(heading) - 1, sha1) &&
+	    preimage[sizeof(heading) + 40 - 1] == '\n' &&
+	    /* does the abbreviated name on the index line agree with it? */
+	    !prefixcmp(preimage + sizeof(heading) - 1, p->old_sha1_prefix))
+		return 0; /* it all looks fine */
+
+	/* we may have full object name on the index line */
+	return get_sha1_hex(p->old_sha1_prefix, sha1);
+}
+
 /* Build an index that contains the just the files needed for a 3way merge */
 static void build_fake_ancestor(struct patch *list, const char *filename)
 {
@@ -3607,8 +3641,10 @@ static void build_fake_ancestor(struct patch *list, const char *filename)
 			continue;
 
 		if (S_ISGITLINK(patch->old_mode)) {
-			if (get_sha1_hex(patch->old_sha1_prefix, sha1))
-				die("submoule change for %s without full index name",
+			if (!preimage_sha1_in_gitlink_patch(patch, sha1))
+				; /* ok, the textual part looks sane */
+			else
+				die("sha1 information is lacking or useless for submoule %s",
 				    name);
 		} else if (!get_sha1_blob(patch->old_sha1_prefix, sha1)) {
 			; /* ok */