git-svn: Fix fetch --no-ignore-externals with GIT_SVN_NO_LIB=1

git-svn: Fix fetch --no-ignore-externals with GIT_SVN_NO_LIB=1

When using Subversion 1.3.1 without Perl bindings (GIT_SVN_NO_LIB=1),
"git-svn fetch --no-ignore-externals" fails with errors like:

  Tree (.../.git/svn/git-svn/tree) is not clean:
  X      directory_with_external

In this case the 'X' lines in the "svn status" output are not a sign
of unclean tree, and therefore should be ignored.

Signed-off-by: Sergey Vlasov <redacted>
Acked-by: Eric Wong <redacted>
Signed-off-by: Junio C Hamano <redacted>