hooks-config: Read 'git config' values from project .hooks-config

Allow projects to configure hooks with a 'git config'-formatted file at
the top of their source tree called ".hooks-config".  This avoids use of
bash-specific syntax and makes configuration declarative.

hooks-config.bash

@@ -20,6 +20,16 @@ if test -z "$GIT_DIR"; then
 fi
 
 # Load hooks configuration from source tree.
+hooks_config=".hooks-config"
+if test -r "$hooks_config"; then
+	hooks_config() {
+		git config -f "$hooks_config" "$@"
+	}
+else
+	hooks_config() {
+		false
+	}
+fi
 config=".hooks-config.bash" && test -r "$config" && . "$config"
 
 # Set up the location for "this" set of hooks.
@@ -28,6 +38,7 @@ HOOKS_DIR="${BASH_SOURCE%/*}"
 hooks_chain() {
 	hook="$1" ; shift
 	chain=$(git config --get hooks.chain-$hook) ||
+	chain="$(hooks_config --get hooks.chain.$hook)" ||
 	eval chain="\${hooks_chain_${hook//-/_}}"
 	test -n "$chain" || return 0
 	case "$chain" in