diff: do not color output when --color=auto and --output=<file> is given

"git diff --output=<file> --color=auto" used to show the ANSI color
sequence in the resulting file when the standard output is connected
to a terminal, because --color=auto check always checks the standard
output, not the actual file that receives the output.

We could correct this by using freopen(3) to redirect the standard
output to the specified file, which is in like with how format-patch
used to match the world order, but following the same reasoning as
the earlier "format-patch: explicitly switch off color when writing
to files", let's be more strict by bypassing the "auto" check when
the --output=<file> option is in use.

Strictly speaking, this is a backwards-incompatible change, but
it is highly unlikely that any user would want to see ANSI color
sequences in a file.

The reason this was not caught earlier is most likely that either
--output=<file> is not used, or only when stdout is redirected
anyway.

Users can still give --color=always if they want a colored diff in
the resulting file.

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

diff.c

@@ -3977,6 +3977,8 @@ int diff_opt_parse(struct diff_options *options,
 		if (!options->file)
 			die_errno("Could not open '%s'", path);
 		options->close_file = 1;
+		if (options->use_color != GIT_COLOR_ALWAYS)
+			options->use_color = GIT_COLOR_NEVER;
 		return argcount;
 	} else
 		return 0;