Add git clone support to sandbox create

Partially address #13 by adding the `--git <repo>` option to `sandbox
create`. This enables the quick cloning of a remote repository instead
of doing the tedious dance of creating a sandbox, cloning the remote
repo, deleting the sandbox repo them moving the `.git` directory before
doing a `git reset --hard HEAD`.

Additionally, improve the error messages for all `sandbox` commands,
clean up the completions a little, and use a consistent style in the
scripts.

sandbox/_sandbox

@@ -1,46 +1,42 @@
 #compdef sandbox
 
-__get_sandboxes() {
-  /bin/ls $SANDBOX_ROOT 2> /dev/null
-}
-
 __sandboxes() {
   local -a sandboxes
-  sandboxes=(${(fo)"$(__get_sandboxes)"})
+  sandboxes=(${(fo)"$(ls $SANDBOX_ROOT 2> /dev/null)"})
   _describe 'in' sandboxes
 }
 
-_sandbox_cmds() {
-  local commands; commands=(
-    'create:Create a new sandbox'
-    'rename:Rename an existing sandbox'
-    'destroy:Destroy an existing sandbox'
-    'list:Show all existing sandboxes'
-    'enable:Enable an existing sandbox'
-    'disable:Disable the current sandbox'
-  )
-  _describe -t commands 'sandbox command' commands "$@"
-}
-
 _sandbox() {
   local context curcontext="$curcontext" state line
   typeset -A opt_args
 
   _arguments -C \
-      '1: :_sandbox_cmds' \
-      '*::arg:->args'
+      '1: :->cmd' \
+      '*:: :->args'
 
   case $state in
+    (cmd)
+      local commands; commands=(
+        'create:Create a new sandbox'
+        'rename:Rename an existing sandbox'
+        'destroy:Destroy an existing sandbox'
+        'list:Show all existing sandboxes'
+        'enable:Enable an existing sandbox'
+        'disable:Disable the current sandbox'
+      )
+      _describe -t commands 'sandbox command' commands "$@"
+      ;;
+
     (args)
       curcontext="${curcontext%:*:*}:sandbox-cmd-$words[1]:"
       case $line[1] in
-        (create|list|disable)
+        (create)
+          _arguments -C '--git[repository to clone]: :'
           ;;
-        (enable|destroy)
+        (rename|enable|destroy)
           _arguments -C '1:: :__sandboxes'
           ;;
-        (rename)
-          _arguments -C '1:: :__sandboxes'
+        (list|disable)
           ;;
       esac
   esac