diff --git a/.hooks/prepare-commit-msg b/.hooks/prepare-commit-msg
--- a/.hooks/prepare-commit-msg
+++ b/.hooks/prepare-commit-msg
@@ -3,7 +3,8 @@
 # prepare-commit-msg: Prepare a commit message upon `git commit` for the
 # user to edit.  A script (rather than a static template) is used, so
 # that we can insert our template text other than at the top of the
-# message.
+# message.  Aims to automatically detect which ports were affected
+# and suggests a commit title.
 #
 # Install by either setting the configuration of the repository to:
 #   git config --add core.hooksPath .hooks
@@ -27,6 +28,42 @@
 
 outfile=$(mktemp /tmp/freebsd-git-commit.XXXXXXXX)
 
+if git rev-parse --verify HEAD >/dev/null 2>&1
+then
+	against=HEAD
+else
+	# Initial commit: diff against an empty tree object
+	against=$(git hash-object -t tree /dev/null)
+fi
+
+PORTS=""
+
+# only determine changed ports if we're not called for amend
+if [ -z "$3" ]; then
+	FILES=$(git diff-index --name-only --cached \
+	    --diff-filter=ACMR $against -- )
+	for f in $FILES; do
+		port=$(dirname $f | egrep "^[a-z]+/[^[:space:]]+$" \
+		    | awk -F/ '{ print $1"/"$2 }')
+		if [ -n "$port" ]; then
+			PORTS="${PORTS} $port"
+		fi
+	done
+fi
+
+if [ -n "$PORTS" ]; then
+	portslist=$(echo $PORTS | tr " " "\n" | sort | uniq)
+	portsnum=$(echo "$portslist" | wc -l)
+	affected_ports=$(echo $(echo "$portslist" | head -n2) \
+	    | sed "s/ /, /g")
+	if [ "$portsnum" -gt 2 ]; then
+		affected_ports="$affected_ports, and $(($portsnum - 2)) more"
+	fi
+	firstline="# Uncomment and add a short description of what changed:
+# $(echo $affected_ports):$(echo " ")
+"
+fi
+
 # Create a commit message template from three parts:
 #
 # 1. The beginning of the git-provided template (up to the first comment-only
@@ -39,7 +76,7 @@
 #    not staged, and untracked files.
 
 cat >"$outfile" <<EOF
-$(awk '1;/^#$/{exit}' "$1")
+$firstline$(awk '1;/^#$/{exit}' "$1")
 # category/port: Subject goes here, max 50 cols -|
 # <then a blank line>
 #                                                         72 columns --|
diff --git a/.arcconfig b/.arcconfig
--- a/.arcconfig
+++ b/.arcconfig
@@ -1,4 +1,5 @@
 {
-	"repository.callsign" : "P",
-	"phabricator.uri" : "https://reviews.freebsd.org/"
+	"phabricator.uri" : "https://reviews.freebsd.org/",
+	"arc.land.onto.default": "main",
+	"arc.land.onto": ["main"]
 }