diff-tree: clean up diff_tree_stdin() function

Split it into the "one commit" vs "two trees" case, since we'll
want to use the "one commit" case for other things too.

diff-tree.c

@@ -342,27 +342,11 @@ static char *generate_header(const char *commit, const char *parent, const char
 	return this_header;
 }
 
-static int diff_tree_stdin(char *line)
+static int diff_tree_commit(const unsigned char *commit, const char *name)
 {
-	int len = strlen(line);
-	unsigned char commit[20], parent[20];
 	unsigned long size, offset;
-	static char this_header[1000];
-	char *buf;
+	char *buf = read_object_with_reference(commit, "commit", &size, NULL);
 
-	if (!len || line[len-1] != '\n')
-		return -1;
-	line[len-1] = 0;
-	if (get_sha1_hex(line, commit))
-		return -1;
-	if (isspace(line[40]) && !get_sha1_hex(line+41, parent)) {
-		line[40] = 0;
-		line[81] = 0;
-		sprintf(this_header, "%s (from %s)\n", line, line+41);
-		header = this_header;
-		return diff_tree_sha1(parent, commit, "");
-	}
-	buf = read_object_with_reference(commit, "commit", &size, NULL);
 	if (!buf)
 		return -1;
 
@@ -372,18 +356,40 @@ static int diff_tree_stdin(char *line)
 			return 0;
 	}
 
-	line[40] = 0;
 	offset = 46;
 	while (offset + 48 < size && !memcmp(buf + offset, "parent ", 7)) {
+		unsigned char parent[20];
 		if (get_sha1_hex(buf + offset + 7, parent))
 			return -1;
-		header = generate_header(line, sha1_to_hex(parent), buf, size);
+		header = generate_header(name, sha1_to_hex(parent), buf, size);
 		diff_tree_sha1(parent, commit, "");
 		if (!header && verbose_header)
 			header_prefix = "\ndiff-tree ";
 		offset += 48;
 	}
-	return -1;
+	return 0;
+}
+
+static int diff_tree_stdin(char *line)
+{
+	int len = strlen(line);
+	unsigned char commit[20], parent[20];
+	static char this_header[1000];
+
+	if (!len || line[len-1] != '\n')
+		return -1;
+	line[len-1] = 0;
+	if (get_sha1_hex(line, commit))
+		return -1;
+	if (isspace(line[40]) && !get_sha1_hex(line+41, parent)) {
+		line[40] = 0;
+		line[81] = 0;
+		sprintf(this_header, "%s (from %s)\n", line, line+41);
+		header = this_header;
+		return diff_tree_sha1(parent, commit, "");
+	}
+	line[40] = 0;
+	return diff_tree_commit(commit, line);
 }
 
 static char *diff_tree_usage =