Use "--ignore-externals" for "svn update" and "svn export"

Use "--ignore-externals" for "svn update" and "svn export"

* svn2svn/svnclient.py (update/remove/export): Introduce wrapper
functions for update/remove/export. For update and remove, pass
"--ignore-externals".

* svn2svn/svnclient.py: Rename private functions to have a leading
underscore.
* svn2svn/svnclient.py: Rename public functions to match svn
sub-command name: get_svn_info -> info, get_svn_status -> status,
get_prop_val -> propget, etc.

* svn2svn/run/svn2svn.py (verify_commit): Remove extraneous
safe_path() call. svnclient.run_svn_log() already handles this.