Add a patch mode for git add --patch

In this new patch mode, every time a line is consumed, we print out an
empty line in its place. This keeps the same number of lines in the
output, and appeases git.

diff-so-fancy

@@ -17,6 +17,7 @@ use warnings FATAL => 'all';
 my $remove_file_add_header     = 1;
 my $remove_file_delete_header  = 1;
 my $clean_permission_changes   = 1;
+my $patch_mode                 = 0;
 my $manually_color_lines       = 0; # Usually git/hg colorizes the lines, but for raw patches we use this
 my $change_hunk_indicators     = git_config_boolean("diff-so-fancy.changeHunkIndicators","true");
 my $strip_leading_indicators   = git_config_boolean("diff-so-fancy.stripLeadingSymbols","true");
@@ -51,6 +52,13 @@ if ($args->{color_on}) {
 	$color_forced         = 1;
 }
 
+# `git add --patch` requries our output to match the number of lines from the
+# input. So, when patch mode is active, we print out empty lines to pad our
+# output to match any lines we've consumed.
+if ($args->{patch}) {
+	$patch_mode = 1;
+}
+
 # We only process ARGV if we don't have STDIN
 if (!$has_stdin) {
 	if ($args->{v} || $args->{version}) {
@@ -178,6 +186,10 @@ sub do_dsf_stuff {
 
 			$last_file_seen =~ s|^\w/||; # Remove a/ (and handle diff.mnemonicPrefix).
 			$in_hunk = 0;
+			if ($patch_mode) {
+				# we are consuming one line, and the debt must be paid
+				print "\n";
+			}
 		########################################
 		# Find the first file: --- a/README.md #
 		########################################
@@ -281,6 +293,9 @@ sub do_dsf_stuff {
 		} elsif ($remove_file_delete_header && $line =~ /^${ansi_color_regex}deleted file mode/) {
 			# Don't print the line (i.e. remove it from the output);
 			$last_file_mode = "delete";
+			if ($patch_mode) {
+				print "\n";
+			}
 		################################
 		# Look for binary file changes #
 		################################
@@ -301,6 +316,10 @@ sub do_dsf_stuff {
 			}
 
 			my ($new_mode) = $next =~ m/new mode (\d+)/;
+
+			if ($patch_mode) {
+				print "\n";
+			}
 			print "$last_file_seen changed file mode from $old_mode to $new_mode\n";
 
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