fix(gen): use absolute paths in generated .gitignore

The generated .gitignore contains relative paths that will match
in both the root of the project or somewhere down the tree.

This can be problematic if the project gets a folder called `dist`
or some other ignored name.

This patch makes the default .gitignores absolute.

Author: huerlisi

templates/common/root/gitignore

@@ -1,5 +1,5 @@
-node_modules
-dist
-.tmp
-.sass-cache
-bower_components
+/node_modules
+/dist
+/.tmp
+/.sass-cache
+/bower_components