difftool regression tests are flaky

[dscho]

The difftool regression tests are flaky. This is not a new issue, it has been like that for a while (and my hunch is that it's an upstream change that introduced the flakiness). But it's annoying, and should be fixed. This is the symptom:

[...]
ok 78 # skip difftool --dir-diff syncs worktree with unstaged change --symlinks (missing SYMLINKS)

expecting success of 7800.79 'difftool --dir-diff syncs worktree without unstaged change --no-symlinks': 
		symlinks=--no-symlinks &&
		
	test_when_finished git reset --hard &&
	git difftool -d $symlinks --extcmd "$PWD/modify-right-file" branch &&
	echo "new content" >expect &&
	test_cmp expect file

	
++ symlinks=--no-symlinks
++ test_when_finished git reset --hard
++ test 0 = 0
++ test_cleanup='{ git reset --hard
		} && (exit "$eval_ret"); eval_ret=$?; :'
++ git difftool -d --no-symlinks --extcmd '/d/a/git-sdk-64/git/t/trash directory.t7800-difftool/modify-right-file' branch
++ echo 'new content'
++ test_cmp expect file
++ test 2 -ne 2
++ eval 'GIT_DIR=/dev/null git diff --no-index --ignore-cr-at-eol --' '"$@"'
+++ GIT_DIR=/dev/null
+++ git diff --no-index --ignore-cr-at-eol -- expect file
diff --git a/expect b/file
index b66ba06..9745c9a 100644
--- a/expect
+++ b/file
@@ -1 +1,3 @@
-new content
+main
+test
+a
error: last command exited with $?=1
++ git reset --hard
HEAD is now at d4705f3 conflict-a
++ exit 1
++ eval_ret=1
++ :
not ok 79 - difftool --dir-diff syncs worktree without unstaged change --no-symlinks
#	
#			symlinks=--no-symlinks &&
#			
#		test_when_finished git reset --hard &&
#		git difftool -d $symlinks --extcmd "$PWD/modify-right-file" branch &&
#		echo "new content" >expect &&
#		test_cmp expect file
#	
#