non-deterministic behaviour of new.ignore (regexp) test

[David Bremner]

For me the following seems to consistently fail after between 30 and 500
attempts

    export NOTMUCH_TEST_QUIET=yes; count=0; while ./T050-new.sh; do (( count++ )); echo $count; done

The test failure looks like

T050-new: Testing "notmuch new" in several variations
 FAIL   Ignore files and directories specified in new.ignore (regexp)
	--- T050-new.25.expected	2018-04-29 02:07:13.055693999 +0000
	+++ T050-new.25.output	2018-04-29 02:07:13.055693999 +0000
	@@ -6,10 +6,6 @@
	 (D) add_files, pass 1: explicitly ignoring /home/bremner/software/upstream/notmuch/test/tmp.T050-new/mail/one/two/ignored_file
	 (D) add_files, pass 1: explicitly ignoring /home/bremner/software/upstream/notmuch/test/tmp.T050-new/mail/one/two/three/.git
	 (D) add_files, pass 1: explicitly ignoring /home/bremner/software/upstream/notmuch/test/tmp.T050-new/mail/one/two/three/ignored_file
	-(D) add_files, pass 2: explicitly ignoring /home/bremner/software/upstream/notmuch/test/tmp.T050-new/mail/.git
	-(D) add_files, pass 2: explicitly ignoring /home/bremner/software/upstream/notmuch/test/tmp.T050-new/mail/.ignored_hidden_file
	-(D) add_files, pass 2: explicitly ignoring /home/bremner/software/upstream/notmuch/test/tmp.T050-new/mail/broken_link
	-(D) add_files, pass 2: explicitly ignoring /home/bremner/software/upstream/notmuch/test/tmp.T050-new/mail/ignored_file
	 (D) add_files, pass 2: explicitly ignoring /home/bremner/software/upstream/notmuch/test/tmp.T050-new/mail/one/ignored_file
	 (D) add_files, pass 2: explicitly ignoring /home/bremner/software/upstream/notmuch/test/tmp.T050-new/mail/one/two/ignored_file
	 (D) add_files, pass 2: explicitly ignoring /home/bremner/software/upstream/notmuch/test/tmp.T050-new/mail/one/two/three/.git