Add simple hook management

Introduces a new user configuration variable `hook-alist', which
specifies which hooks to link to which executables, for example:

,----
| (set! hook-alist '(("commit-msg" . "/path/to/my/commit/msg/hook")))
`----

With this setting the command `config hooks' will install
`/path/to/my/commit/msg/hook' to the `commit-msg' hook of each
repository not in the `config-exclusion-list' setting.

gitto/config.scm

@@ -21,12 +21,24 @@
   #:use-module (ice-9 format)
   #:use-module (ice-9 rdelim)
   #:export (global-config
+            hook-alist
 
+            install-hooks
             merge-config
             read-config
             write-config))
 
 (define global-config '())
+(define hook-alist '())
+
+(define (install-hooks repo-location)
+  (for-each
+   (lambda (hook)
+     (let ((new-name (string-append repo-location "/.git/hooks/"
+                                    (car hook))))
+       (unless (file-exists? new-name)
+         (symlink (cdr hook) new-name))))
+   hook-alist))
 
 (define (merge-config repo-name x y)
   (let ((lst (if x (list-copy x) '())))

gitto/main.scm

@@ -65,6 +65,7 @@ gitto [command [arguments ...]]
   config global        Show template configuration
   config update        Merge template configuration with each
                        repository's configuration
+  config hooks         Install configured hooks for repositories
   version              Display version
   help                 Display this help
 "))
@@ -180,7 +181,12 @@ gitto [command [arguments ...]]
                 (newline))
               repositories))
    ((equal? (car args) "global") (show-global-config))
-   ((equal? (car args) "update") (update-config))))
+   ((equal? (car args) "update") (update-config))
+   ((equal? (car args) "hooks")
+    (for-each (lambda (r)
+                (unless (member (repo-name r) config-exclusion-list)
+                  (install-hooks (repo-location r))))
+              repositories))))
 (set! command-list (append command-list `(("config" . ,show-config))))
 
 (define (update-repo-config repo)