Keep each release note in a single file

To avoid annoying merge conflicts.

Perl script is provided for convenience and simple format checking in
PRs.

release-notes/generator.pl

@@ -0,0 +1,89 @@
+#!/usr/bin/env perl
+use strict;
+use warnings;
+use v5.10;
+use Getopt::Long;
+
+my @template = (
+'
+ ┌──────────────────────────────┐
+ │ Release notes for i3 v4.21   │
+ └──────────────────────────────┘
+
+This is i3 v4.21. This version is considered stable. All users of i3 are
+strongly encouraged to upgrade.
+
+
+ ┌────────────────────────────┐
+ │ Changes in i3 v4.21        │
+ └────────────────────────────┘
+
+',
+'
+ ┌────────────────────────────┐
+ │ Bugfixes                   │
+ └────────────────────────────┘
+
+');
+
+my $print_urls = 0;
+my $result = GetOptions('print-urls' => \$print_urls);
+
+sub read_changefiles {
+    my $dirpath = shift;
+    opendir my $dir, $dirpath or die "Cannot open directory $dirpath: $!";
+    my @files = readdir $dir;
+    closedir $dir;
+
+    my $s = '';
+    for my $filename (@files) {
+        next if $filename eq '.';
+        next if $filename eq '..';
+        next if $filename eq '0-example';
+
+        die "Filename $filename should start with a number (e.g. the pull request number)" unless ($filename =~ /^\d+/);
+
+        $filename = $dirpath . '/' . $filename;
+        open my $in, '<', $filename or die "can't open $filename: $!";
+        my @lines = <$in>;
+        close $in or die "can't close $filename: $!";
+
+        my $content = trim(join("\n  ", map { trim($_) } @lines));
+        die "$filename can't be empty" unless length($content) > 0;
+
+        my $commit = `git log --diff-filter=A --pretty=format:"%H" $filename`;
+        $commit = trim($commit) if defined($commit);
+        die "$filename: git log failed to find commit" if ($?) || (length($commit) == 0);
+
+        my $url = '';
+        if ($print_urls) {
+            my $pr = find_pr($commit);
+            my $url = 'https://github.com/i3/i3/commit/' . $commit;
+            $url = 'https://github.com/i3/i3/pull/' . $pr if defined($pr);
+            $url = $url . "\n";
+        }
+
+        $s = $s . '  • ' . $content . "\n" . $url;
+    }
+    return $s;
+}
+
+sub find_pr {
+    my $hash = shift;
+    my $result = `git log --merges --ancestry-path --oneline $hash..next | grep 'Merge pull request' | tail -n1`;
+    return unless defined($result);
+
+    return unless ($result =~ /Merge pull request .([0-9]+)/);
+    return $1;
+}
+
+sub trim {
+    (my $s = $_[0]) =~ s/^\s+|\s+$//g;
+    return $s;
+}
+
+# Expected to run for i3's git root
+my $changes = read_changefiles('release-notes/changes');
+my $bugfixes = read_changefiles('release-notes/bugfixes');
+
+print $template[0] . $changes . $template[1] . $bugfixes;