add quieter versions of parse_{tree,commit}

When we call parse_commit, it will complain to stderr if the
object does not exist or cannot be read. This means that we
may produce useless error messages if this situation is
expected (e.g., because the object is marked UNINTERESTING,
or because revs->ignore_missing_links is set).

We can fix this by adding a new "parse_X_gently" form that
takes a flag to suppress the messages. The existing
"parse_X" form is already gentle in the sense that it
returns an error rather than dying, and we could in theory
just add a "quiet" flag to it (with existing callers passing
"0"). But doing it this way means we do not have to disturb
existing callers.

Note also that the new flag is "quiet_on_missing", and not
just "quiet". We could add a flag to suppress _all_ errors,
but besides being a more invasive change (we would have to
pass the flag down to sub-functions, too), there is a good
reason not to: we would never want to use it. Missing a
linked object is expected in some circumstances, but it is
never expected to have a malformed commit, or to get a tree
when we wanted a commit.  We should always complain about
these corruptions.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.c

@@ -353,7 +353,7 @@ int parse_commit_buffer(struct commit *item, const void *buffer, unsigned long s
 	return 0;
 }
 
-int parse_commit(struct commit *item)
+int parse_commit_gently(struct commit *item, int quiet_on_missing)
 {
 	enum object_type type;
 	void *buffer;
@@ -366,7 +366,8 @@ int parse_commit(struct commit *item)
 		return 0;
 	buffer = read_sha1_file(item->object.sha1, &type, &size);
 	if (!buffer)
-		return error("Could not read %s",
+		return quiet_on_missing ? -1 :
+			error("Could not read %s",
 			     sha1_to_hex(item->object.sha1));
 	if (type != OBJ_COMMIT) {
 		free(buffer);

commit.h

@@ -59,7 +59,11 @@ struct commit *lookup_commit_reference_by_name(const char *name);
 struct commit *lookup_commit_or_die(const unsigned char *sha1, const char *ref_name);
 
 int parse_commit_buffer(struct commit *item, const void *buffer, unsigned long size);
-int parse_commit(struct commit *item);
+int parse_commit_gently(struct commit *item, int quiet_on_missing);
+static inline int parse_commit(struct commit *item)
+{
+	return parse_commit_gently(item, 0);
+}
 void parse_commit_or_die(struct commit *item);
 
 /*

tree.c

@@ -198,7 +198,7 @@ int parse_tree_buffer(struct tree *item, void *buffer, unsigned long size)
 	return 0;
 }
 
-int parse_tree(struct tree *item)
+int parse_tree_gently(struct tree *item, int quiet_on_missing)
 {
 	 enum object_type type;
 	 void *buffer;
@@ -208,7 +208,8 @@ int parse_tree(struct tree *item)
 		return 0;
 	buffer = read_sha1_file(item->object.sha1, &type, &size);
 	if (!buffer)
-		return error("Could not read %s",
+		return quiet_on_missing ? -1 :
+			error("Could not read %s",
 			     sha1_to_hex(item->object.sha1));
 	if (type != OBJ_TREE) {
 		free(buffer);

tree.h

@@ -15,7 +15,11 @@ struct tree *lookup_tree(const unsigned char *sha1);
 
 int parse_tree_buffer(struct tree *item, void *buffer, unsigned long size);
 
-int parse_tree(struct tree *tree);
+int parse_tree_gently(struct tree *tree, int quiet_on_missing);
+static inline int parse_tree(struct tree *tree)
+{
+	return parse_tree_gently(tree, 0);
+}
 void free_tree_buffer(struct tree *tree);
 
 /* Parses and returns the tree in the given ent, chasing tags and commits. */