chore(script/prepare-commit-msg): add simple commit message generator

doc/commit_convention.md

@@ -76,3 +76,16 @@ but contains
 The compiler does not generate an error message. It silently uses the
 operator bool() to coerce the expression into a Boolean. This produces
 counter-intuitive behavior, and may confuse developers.
+
+"Prepare commit message" git hook
+=================================
+
+Execute the following to activate a little script that tries to synthesize
+the first part of the commit message:
+
+```bash
+ln -s script/prepare-commit-msg .git/hooks
+```
+
+Currently, if just a single file has changed, the hook will use it for the scope
+and default the change type to "feat".

script/prepare-commit-msg

@@ -0,0 +1,13 @@
+#!/bin/bash
+set -euo pipefail
+
+# exit on existing commit message
+[[ -z "$2" ]] || exit 0
+
+diff=$(git diff --staged --stat | head -n -1)
+[[ $(echo "$diff" | wc -l) == 1 ]] || exit 0
+
+file=$(echo "$diff" | cut -f 2 -d' ')
+orig=$(cat "$1")
+echo "feat($file):" > $1
+echo "$orig" >> $1