format-patch: tie file-opening logic to output_directory

format-patch: tie file-opening logic to output_directory

In format-patch we're either outputting to stdout or to individual files
in an output directory (which may be just "./"). Our logic for whether
to open a new file for each patch is checked with "!use_stdout", but it
is equally correct to check for a non-NULL output_directory.

The distinction will matter when we add a new single-stream output in a
future patch, when only one of the three methods will want individual
files. Let's swap the logic here in preparation.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>