Remove some instances where a RewriteBase must be specified
Previously, any time you used a relative substitution in per-directory/htaccess context, you needed to specify a RewriteBase. But in case where the context document root and context prefix are known via e.g. mod_userdir or mod_alias, and the substitution is under the context document root, we can determine the replacement automatically.
This makes htaccess files or config snippets a bit more portable.