git-svn: fix segfaults from accessing svn_log_changed_path_t

git-svn: fix segfaults from accessing svn_log_changed_path_t

svn_log_changed_path_t structs were being used out of scope
outside of svn_ra_get_log (because I wanted to eventually be
able to use git-svn with only a single connection to the
repository).  So now we dup them into a hash.

This was fixed while making --follow-parent fetches more
efficient.  I've moved parsing of the command-line --revision
argument outside of the Git::SVN module so Git::SVN::fetch() can
be used in more places (such as find_parent_branch).

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