[__git] Implement onchange triggers and messaging.

Closes #820.

cdist/conf/type/__git/gencode-remote

@@ -71,16 +71,24 @@ EOF
             fi
             # Actually clone the repository
             su -m "$git_user" -c "git clone --quiet '$recursive' '$shallow' --branch '$branch' '$source' '$destination'"
+            echo create >> "$__messages_out"
+            repo_changed="YES"
         elif [ "$branch_should" != "$branch_is" ]; then
             # User has changed tag / branch, let's update that
             echo su -m "$git_user" -c "git checkout '$branch_should'"
+            echo "change $branch_is -> $branch_should" >> "$__messages_out"
+            repo_changed="YES"
         elif [ "$needs_update" = "YES" ]; then
             # The remote has newer information than our repository.
             # Fetch was done in the explorer, here we can just pull
             echo su -m "$git_user" -c "git -C '$destination' pull"
+            echo update >> "$__messages_out"
+            repo_changed="YES"
         fi
         if [ -n "$needs_chown" ]; then
             echo chown -R "${owner}:${group}" "$destination"
+            echo "chown -R '${owner}:${group}'" >> "$__messages_out"
+            repo_changed="YES"
         fi
         if [ -f "$__object/parameter/mode-recursive" ]; then
             mode_recursive="-R"
@@ -91,7 +99,11 @@ EOF
     ;;
 
     absent)
-        # Handled in manifest
+        # Handled in manifest, except for the change triggers
+        if [ "$state_should" != "$state_is" ]; then
+            echo remove >> "$__messages_out"
+            repo_changed="YES"
+        fi
     ;;
 
     *)
@@ -99,3 +111,7 @@ EOF
         exit 1
     ;;
 esac
+
+if [ -n "$repo_changed" ]; then
+    cat "$__object/parameter/onchange"
+fi

cdist/conf/type/__git/man.rst

@@ -35,6 +35,9 @@ mode
    By default it only applies to the top-level directory.
    See the mode-recursive parameter as well.
 
+onchange
+   The code to run if the repository is first-cloned, changes or is removed.
+
 owner
    User to chown to.
 
@@ -50,6 +53,18 @@ recursive
 shallow
    Sets --depth=1 and --shallow-submodules for cloning repositories with big history.
 
+MESSAGES
+--------
+change <old_branch> -> <new_branch>
+    The directory tracks old_branch but should track new_branch, it will be changed
+chown -R <owner>:<group>
+    Changed ownership
+create
+    Freshly created the directory with the repository clone
+remove
+    The directory with the repository exists, but state is absent, it will be removed
+update
+    The repository tracks a branch that has been updated on the remote
 
 EXAMPLES
 --------

cdist/conf/type/__git/parameter/optional

@@ -1,5 +1,6 @@
 state
 branch
 group
+onchange
 owner
 mode