diff-highlight: fetch all config with one process

When diff-highlight was written, there was no way to fetch multiple
config keys _and_ have them interpreted as colors. So we were stuck
with either invoking git-config once for each config key, or fetching
them all and converting human-readable color names into ANSI codes
ourselves.

I chose the former, but it means that diff-highlight kicks off 6
git-config processes (even if you haven't configured anything, it has to
check each one).

But since Git 2.18.0, we can do:

   git config --type=color --get-regexp=^color\.diff-highlight\.

to get all of them in one shot.

Note that any callers which pass in colors directly to the module via
@OLD_HIGHLIGHT and @NEW_HIGHLIGHT (like diff-so-fancy plans to do) are
unaffected; those colors suppress any config lookup we'd do ourselves.

You can see the effect like:

  # diff-highlight suppresses git-config's stderr, so dump
  # trace through descriptor 3
  git show d1f33c7 | GIT_TRACE=3 diff-highlight 3>&2 >/dev/null

which drops from 6 lines down to 1.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: peff

contrib/diff-highlight/DiffHighlight.pm

@@ -131,9 +131,21 @@ sub highlight_stdin {
 # of it being used in other settings. Let's handle our own
 # fallback, which means we will work even if git can't be run.
 sub color_config {
+	our $cached_config;
 	my ($key, $default) = @_;
-	my $s = `git config --get-color $key 2>$NULL`;
-	return length($s) ? $s : $default;
+
+	if (!defined $cached_config) {
+		$cached_config = {};
+		my $data = `git config --type=color --get-regexp '^color\.diff-highlight\.' 2>$NULL`;
+		for my $line (split /\n/, $data) {
+			my ($key, $color) = split ' ', $line, 2;
+			$key =~ s/^color\.diff-highlight\.// or next;
+			$cached_config->{$key} = $color;
+		}
+	}
+
+	my $s = $cached_config->{$key};
+	return defined($s) ? $s : $default;
 }
 
 sub show_hunk {
@@ -172,16 +184,16 @@ sub load_color_config {
 	# always be set if you want highlighting to do anything.
 	if (!defined $OLD_HIGHLIGHT[1]) {
 		@OLD_HIGHLIGHT = (
-			color_config('color.diff-highlight.oldnormal'),
-			color_config('color.diff-highlight.oldhighlight', "\x1b[7m"),
-			color_config('color.diff-highlight.oldreset', "\x1b[27m")
+			color_config('oldnormal'),
+			color_config('oldhighlight', "\x1b[7m"),
+			color_config('oldreset', "\x1b[27m")
 		);
 	}
 	if (!defined $NEW_HIGHLIGHT[1]) {
 		@NEW_HIGHLIGHT = (
-			color_config('color.diff-highlight.newnormal', $OLD_HIGHLIGHT[0]),
-			color_config('color.diff-highlight.newhighlight', $OLD_HIGHLIGHT[1]),
-			color_config('color.diff-highlight.newreset', $OLD_HIGHLIGHT[2])
+			color_config('newnormal', $OLD_HIGHLIGHT[0]),
+			color_config('newhighlight', $OLD_HIGHLIGHT[1]),
+			color_config('newreset', $OLD_HIGHLIGHT[2])
 		);
 	};
 }