git svn: fix --revision when fetching deleted paths

git svn: fix --revision when fetching deleted paths

When using the -r/--revision argument to fetch deleted history,
calling SVN::Ra::get_log() from an SVN::Ra object initialized
to track the deleted URL will fail.

This regression was introduced in:
  commit 4aacaeb3dc82bb6479e70e120053dc27a399460e
  "fix shallow clone when upstream revision is too new"

We now ignore errors from SVN::Ra::get_log() here because using
--revision will always override the value of $head here if
(and only if) we're tracking deleted directories.

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