filebuf: add option not to buffer the contents at all

The new indexer needs to be able to bypass any kind of buffering, as
it's trying to map data that it has just written to disk.

src/filebuf.c

@@ -194,14 +194,19 @@ int git_filebuf_open(git_filebuf *file, const char *path, int flags)
 
 	memset(file, 0x0, sizeof(git_filebuf));
 
+	if (flags & GIT_FILEBUF_DO_NOT_BUFFER)
+		file->do_not_buffer = true;
+
 	file->buf_size = WRITE_BUFFER_SIZE;
 	file->buf_pos = 0;
 	file->fd = -1;
 	file->last_error = BUFERR_OK;
 
 	/* Allocate the main cache buffer */
+	if (!file->do_not_buffer) {
 		file->buffer = git__malloc(file->buf_size);
 		GITERR_CHECK_ALLOC(file->buffer);
+	}
 
 	/* If we are hashing on-write, allocate a new hash context */
 	if (flags & GIT_FILEBUF_HASH_CONTENTS) {
@@ -345,6 +350,9 @@ int git_filebuf_write(git_filebuf *file, const void *buff, size_t len)
 
 	ENSURE_BUF_OK(file);
 
+	if (file->do_not_buffer)
+		return file->write(file, (void *)buff, len);
+
 	for (;;) {
 		size_t space_left = file->buf_size - file->buf_pos;
 

src/filebuf.h

@@ -19,7 +19,8 @@
 #define GIT_FILEBUF_APPEND				(1 << 2)
 #define GIT_FILEBUF_FORCE				(1 << 3)
 #define GIT_FILEBUF_TEMPORARY			(1 << 4)
-#define GIT_FILEBUF_DEFLATE_SHIFT		(5)
+#define GIT_FILEBUF_DO_NOT_BUFFER		(1 << 5)
+#define GIT_FILEBUF_DEFLATE_SHIFT		(6)
 
 #define GIT_FILELOCK_EXTENSION ".lock\0"
 #define GIT_FILELOCK_EXTLENGTH 6
@@ -41,6 +42,7 @@ struct git_filebuf {
 	size_t buf_size, buf_pos;
 	git_file fd;
 	bool fd_is_open;
+	bool do_not_buffer;
 	int last_error;
 };