.github: unwrap ChangeLog paragraphs in release notes

The ChangeLog is hard-wrapped at 72 columns, which the GitHub releases
page renders verbatim as a ragged right edge.  Join the continuation
lines of each paragraph and list item, as already done in Infix, and
let the browser do the wrapping.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

.github/scripts/extract-changelog.sh

@@ -0,0 +1,29 @@
+#!/bin/sh
+#
+# Extract the topmost release entry from doc/ChangeLog.md and unwrap
+# its paragraphs, so the GitHub releases page can wrap them to the
+# browser width instead of showing the hard 72-column breaks.
+#
+# Usage: .github/scripts/extract-changelog.sh < doc/ChangeLog.md > release.md
+#
+set -e
+
+# The entry sits between the first two underlines, minus the blank
+# line separating it from the next heading.
+awk '/^-----*$/{if (x == 1) exit; x=1;next}x' | head -n -1 |
+
+# Blank lines, headings, blockquotes, tables, and list items (at any
+# indent) start a new block, everything else continues the current one.
+awk '
+function flush() { if (buf) print buf; buf = "" }
+{
+    if (/^$/) { flush(); print; blank = 1; next }
+    if (/^ *([-*] |[0-9]+\. |[>#|])/ || blank || !buf || brk)
+        { flush(); buf = $0 }
+    else
+        { sub(/^ +/, ""); buf = buf " " $0 }
+    blank = 0
+    brk = /  $/
+}
+END { flush() }
+'