Add comment re: why SHA1 is disabled

contrib/verify-commits/gpg.sh

@@ -11,6 +11,15 @@ IFS='
 if [ "$BITCOIN_VERIFY_COMMITS_ALLOW_SHA1" = 1 ]; then
 	GPG_RES="$(echo "$INPUT" | gpg --trust-model always "$@" 2>/dev/null)"
 else
+	# Note how we've disabled SHA1 with the --weak-digest option, disabling
+	# signatures - including selfsigs - that use SHA1. While you might think that
+	# collision attacks shouldn't be an issue as they'd be an attack on yourself,
+	# in fact because what's being signed is a commit object that's
+	# semi-deterministically generated by untrusted input (the pull-req) in theory
+	# an attacker could construct a pull-req that results in a commit object that
+	# they've created a collision for. Not the most likely attack, but preventing
+	# it is pretty easy so we do so as a "belt-and-suspenders" measure.
+
 	GPG_RES="$(echo "$INPUT" | gpg --trust-model always --weak-digest sha1 "$@" 2>/dev/null)"
 fi
 for LINE in $(echo "$GPG_RES"); do