Remove some instances where a RewriteBase must be specified

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.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1642484 13f79535-47bb-0310-9956-ffa450edef68