Fix possible coredump with fast-import --import-marks

When e8438420bb allowed us to reload
the marks table on subsequent runs of fast-import we really broke
things, as we set pack_id to MAX_PACK_ID for any objects we imported
into the marks table.  Creating a branch from that mark should fail
as we attempt to read the object through a non-existant packed_git
pointer.  Instead we have to use the normal Git object system to
locate the older commit, as we ourselves do not have a reference
to the packed_git it resides in.

This bug only occurred because t9300 was not complete enough.
When we added the --import-marks feature we didn't actually test
its implementation enough to verify the function worked as intended.
I have corrected that, and included the changes as part of this fix.
Prior versions of fast-import fail the new test(s); this commit
allows them to pass.

Credit for this bug find goes to Simon Hausmann <simon@lst.de> as
he recently identified a similiar bug in the tree lazy-loading path.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

fast-import.c

@@ -1709,14 +1709,16 @@ static void cmd_from(struct branch *b)
 	} else if (*from == ':') {
 		uintmax_t idnum = strtoumax(from + 1, NULL, 10);
 		struct object_entry *oe = find_mark(idnum);
-		unsigned long size;
-		char *buf;
 		if (oe->type != OBJ_COMMIT)
 			die("Mark :%" PRIuMAX " not a commit", idnum);
 		hashcpy(b->sha1, oe->sha1);
-		buf = gfi_unpack_entry(oe, &size);
-		cmd_from_commit(b, buf, size);
-		free(buf);
+		if (oe->pack_id != MAX_PACK_ID) {
+			unsigned long size;
+			char *buf = gfi_unpack_entry(oe, &size);
+			cmd_from_commit(b, buf, size);
+			free(buf);
+		} else
+			cmd_from_existing(b);
 	} else if (!get_sha1(from, b->sha1))
 		cmd_from_existing(b);
 	else

t/t9300-fast-import.sh

@@ -119,6 +119,35 @@ test_expect_success \
 		</dev/null &&
 	git diff -u expect marks.new'
 
+test_tick
+cat >input <<INPUT_END
+commit refs/heads/verify--import-marks
+committer $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL> $GIT_COMMITTER_DATE
+data <<COMMIT
+recreate from :5
+COMMIT
+
+from :5
+M 755 :2 copy-of-file2
+
+INPUT_END
+test_expect_success \
+	'A: verify marks import does not crash' \
+	'git-fast-import --import-marks=marks.out <input &&
+	 git-whatchanged verify--import-marks'
+test_expect_success \
+	'A: verify pack' \
+	'for p in .git/objects/pack/*.pack;do git-verify-pack $p||exit;done'
+cat >expect <<EOF
+:000000 100755 0000000000000000000000000000000000000000 7123f7f44e39be127c5eb701e5968176ee9d78b1 A	copy-of-file2
+EOF
+git-diff-tree -M -r master verify--import-marks >actual
+test_expect_success \
+	'A: verify diff' \
+	'compare_diff_raw expect actual &&
+	 test `git-rev-parse --verify master:file2` \
+	    = `git-rev-parse --verify verify--import-marks:copy-of-file2`'
+
 ###
 ### series B
 ###