git-svn: canonicalize newly-minted URLs

git-svn: canonicalize newly-minted URLs

Go through all the spots that use the new add_path_to_url() to
make a new URL and canonicalize them.

* copyfrom_path has to be canonicalized else find_parent_branch
  will get confused

* due to the `canonicalize_url($full_url) ne $full_url)` line of
  logic in gs_do_switch(), $full_url is left alone until after.

At this point SVN 1.7 passes except for 3 tests in
t9100-git-svn-basic.sh that look like an SVN bug to do with
symlinks.

[ew: commit title]

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