do not check odb_mkstemp return value for errors

The odb_mkstemp function does not return an error; it dies
on failure instead. But many of its callers compare the
resulting descriptor against -1 and die themselves.

Mostly this is just pointless, but it does raise a question
when looking at the callers: if they show the results of the
"template" buffer after a failure, what's in it? The answer
is: it doesn't matter, because it cannot happen.

So let's make that clear by removing the bogus error checks.
In bitmap_writer_finish(), we can drop the error-handling
code entirely. In the other two cases, it's shared with the
open() in another code path; we can just move the
error-check next to that open() call.

And while we're at it, let's flesh out the function's
docstring a bit to make the error behavior clear.

Signed-off-by: Jeff King <peff@peff.net>

pack-bitmap-write.c

@@ -517,8 +517,6 @@ void bitmap_writer_finish(struct pack_idx_entry **index,
 
 	int fd = odb_mkstemp(tmp_file, sizeof(tmp_file), "pack/tmp_bitmap_XXXXXX");
 
-	if (fd < 0)
-		die_errno("unable to create '%s'", tmp_file);
 	f = sha1fd(fd, tmp_file);
 
 	memcpy(header.magic, BITMAP_IDX_SIGNATURE, sizeof(BITMAP_IDX_SIGNATURE));