t7800-difftool: don't accidentally match tmp dirs

t7800-difftool: don't accidentally match tmp dirs

In a bunch of test cases in 't7800-difftool.sh' we 'grep' for specific
filenames in 'git difftool's output, and those test cases are prone to
occasional failures because those filenames might be part of the name
of difftool's temporary directory as well, e.g.:

  +git difftool --dir-diff --no-symlinks --extcmd ls v1
  +grep sub output
  +test_line_count = 2 sub-output
  test_line_count: line count for sub-output != 2
  /tmp/git-difftool.Ssubfq/left/:
  sub
  /tmp/git-difftool.Ssubfq/right/:
  sub
  error: last command exited with $?=1
  not ok 50 - difftool --dir-diff v1 from subdirectory --no-symlinks

Fix this by tightening the 'grep' patterns looking for those
interesting filenames to match only lines where a filename stands on
its own.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>