fast-export: Allow pruned-references in mark file

fast-export can fail because of some pruned-reference when importing a
mark file.

The problem happens in the following scenario:

    $ git fast-export --export-marks=MARKS master
    (rewrite master)
    $ git prune
    $ git fast-export --import-marks=MARKS master

This might fail if some references have been removed by prune
because some marks will refer to no longer existing commits.
git-fast-export will not need these objects anyway as they were no
longer reachable.

We still need to update last_numid so we don't change the mapping
between marks and objects for remote-helpers.
Unfortunately, the mark file should not be rewritten without lost marks
if no new objects has been exported, as we could lose track of the last
last_numid.

Signed-off-by: Antoine Pelisse <apelisse@gmail.com>
Reviewed-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-fast-export.txt

@@ -66,6 +66,8 @@ produced incorrect results if you gave these options.
 	incremental runs.  As <file> is only opened and truncated
 	at completion, the same path can also be safely given to
 	\--import-marks.
+	The file will not be written if no new object has been
+	marked/exported.
 
 --import-marks=<file>::
 	Before processing any input, load the marks specified in

builtin/fast-export.c

@@ -618,9 +618,12 @@ static void import_marks(char *input_file)
 			|| *mark_end != ' ' || get_sha1(mark_end + 1, sha1))
 			die("corrupt mark line: %s", line);
 
+		if (last_idnum < mark)
+			last_idnum = mark;
+
 		object = parse_object(sha1);
 		if (!object)
-			die ("Could not read blob %s", sha1_to_hex(sha1));
+			continue;
 
 		if (object->flags & SHOWN)
 			error("Object %s already has a mark", sha1_to_hex(sha1));
@@ -630,8 +633,6 @@ static void import_marks(char *input_file)
 			continue;
 
 		mark_object(object, mark);
-		if (last_idnum < mark)
-			last_idnum = mark;
 
 		object->flags |= SHOWN;
 	}
@@ -645,6 +646,7 @@ int cmd_fast_export(int argc, const char **argv, const char *prefix)
 	struct string_list extra_refs = STRING_LIST_INIT_NODUP;
 	struct commit *commit;
 	char *export_filename = NULL, *import_filename = NULL;
+	uint32_t lastimportid;
 	struct option options[] = {
 		OPT_INTEGER(0, "progress", &progress,
 			    N_("show progress after <n> objects")),
@@ -688,6 +690,7 @@ int cmd_fast_export(int argc, const char **argv, const char *prefix)
 
 	if (import_filename)
 		import_marks(import_filename);
+	lastimportid = last_idnum;
 
 	if (import_filename && revs.prune_data.nr)
 		full_tree = 1;
@@ -710,7 +713,7 @@ int cmd_fast_export(int argc, const char **argv, const char *prefix)
 
 	handle_tags_and_duplicates(&extra_refs);
 
-	if (export_filename)
+	if (export_filename && lastimportid != last_idnum)
 		export_marks(export_filename);
 
 	if (use_done_feature)