Implement hook system

Thanks to Vincent Demeester for this idea.

doc/vcsh.1.ronn

@@ -111,6 +111,21 @@ an interactive user.
 As noted earlier, `vcsh` will set <$GIT_DIR> and <$GIT_WORK_TREE> to the
 appropriate values for fake bare git repositories.
 
+## HOOK SYSTEM
+
+`vcsh` provides a hook system. Hook scripts need to be executable and placed
+in <$XDG_CONFIG_HOME/vcsh/hooks-available>. From there, they should be
+soft-linked <$XDG_CONFIG_HOME/vcsh/hooks-enabled>.
+
+Hooks follow a simple format. `pre-run` will be run before anything is run.
+If you want to have more than one script for a certain hook, just append
+strings to order them. A system of `pre-run`, `pre-run.10`, `pre-run.20` etc
+is suggested. The dot it optional.
+
+If you want to create hooks for a specific `vcsh` repository, simply prepend
+the repository's name, followed by a dot, i.e. `zsh.pre-run`. This dot is
+mandatory.
+
 ## DETAILED HOWTO AND FURTHER READING
 
 Man pages are intended to be short and thus often useless to glean best

vcsh

@@ -11,6 +11,7 @@
 [ -r "$XDG_CONFIG_HOME/vcsh/config" ] && . "$XDG_CONFIG_HOME/vcsh/config"
 [ -n "$VCSH_DEBUG" ]                  && set -vx
 [ -z "$VCSH_REPO_D" ]                 && VCSH_REPO_D="$XDG_CONFIG_HOME/vcsh/repo.d"
+[ -z "$VCSH_HOOK_D" ]                 && VCSH_HOOK_D="$XDG_CONFIG_HOME/vcsh/hooks-enabled"
 [ -z "$VCSH_BASE" ]                   && VCSH_BASE="$HOME"
 [ -z "$VCSH_GITIGNORE" ]              && VCSH_GITIGNORE='exact'
 
@@ -103,14 +104,23 @@ To continue, type \"Yes, do as I say\""
 }
 
 enter() {
+	hook pre-enter
 	use
 	$SHELL
+	hook post-enter
 }
 
 git_dir_exists() {
 	[ -d "$GIT_DIR" ] || fatal "no repository found for '$VCSH_REPO_NAME'" 12
 }
 
+hook() {
+	for f in $VCSH_HOOK_D/$1* $VCSH_HOOK_D/$VCSH_REPO_NAME.$1*; do
+		[ -x "$f" ] || continue
+		"$f"
+	done
+}
+
 init() {
 	[ ! -e "$GIT_DIR" ] || fatal "'$GIT_DIR' exists" 10
 	export GIT_WORK_TREE="$VCSH_BASE"
@@ -150,8 +160,10 @@ rename() {
 }
 
 run() {
+	hook pre-run
 	use
 	$VCSH_EXTERNAL_COMMAND
+	hook post-run
 }
 
 setup() {