format-patch: explicitly switch off color when writing to files

The --color=auto handling is done by seeing if file descriptor 1
(the standard output) is connected to a terminal.  format-patch
used freopen() to reuse the standard output stream even when sending
its output to an on-disk file, and this check is appropriate.

In the next step, however, we will stop reusing "FILE *stdout", and
instead start using arbitrary file descriptor obtained by doing an
fopen(3) ourselves.  The check --color=auto does will become useless,
as we no longer are writing to the standard output stream.

But then, we do not need to guess to begin with. As argued in the commit
message of 7787570c (format-patch: ignore ui.color, 2011-09-13), we do not
allow the ui.color setting to affect format-patch's output. The only time,
therefore, that we allow color sequences to be written to the output files
is when the user specified the --color=always command-line option explicitly.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/log.c

@@ -1578,6 +1578,8 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 		setup_pager();
 
 	if (output_directory) {
+		if (rev.diffopt.use_color != GIT_COLOR_ALWAYS)
+			rev.diffopt.use_color = GIT_COLOR_NEVER;
 		if (use_stdout)
 			die(_("standard output, or directory, which one?"));
 		if (mkdir(output_directory, 0777) < 0 && errno != EEXIST)