git-svn: introduce add_path_to_url function

git-svn: introduce add_path_to_url function

Remove the ad-hoc versions.

This is mostly to normalize the process and ensure the URLs produced
don't have double slashes or anything.

Also provides a place to fix the corner case where a file path
contains a percent sign.

[ew: commit title]

Signed-off-by: Eric Wong <normalperson@yhbt.net>