Support files in "svn:externals" property in SVN 1.6+ working copies

Summary:
These changes allow detection of files, that come from folder's "svn:externals" property. Before only folders were properly detected.

Fixes T6447

Test Plan:
# create 2 svn repositories (internally these can be 2 folders on same SVN server of course)
# configure 2nd folder to include one folder and one file (try binary file, like "favicon.ico") via "svn:externals" property
# checkout 2nd folder
# try running `arc diff`

Before this would result in UTF-8 encoding error on binary file that came from "svn:externals". Now file will be properly skipped.

Reviewers: #blessed_reviewers

Subscribers: epriestley

Maniphest Tasks: T6447

Differential Revision: https://secure.phabricator.com/D10772
Answering "N" to external change prompt will include externals in created diff

See https://secure.phabricator.com/T6446
Merge branches 'T6446' and 'T6447'

* T6446:
  Answering "N" to external change prompt will include externals in created diff

* T6447:
  Support files in "svn:externals" property in SVN 1.6+ working copies