t7800: don't rely on reuse_worktree_file()

A test in t7800 tries to make sure that when git-difftool runs an external tool that fails, it stops looking at files. Our fake failing tool prints the file name it was asked to diff before exiting non-zero, and then we confirm the output contains only that file. However, this subtly relies on our internal reuse_worktree_file(). Because we're diffing between branches, the command run by difftool might see: - the git-stored filename (e.g., "file"), if we decided that the working tree contents were up-to-date with the object in the index and HEAD, and we could reuse them - a temporary filename (e.g. "/tmp/abc123_file") if we had to dump the contents from the object database If the latter case happens, then the test fails, because it's expecting the string "file". I discovered this when debugging something unrelated with reuse_worktree_file(). I _thought_ it should be able to be triggered by a racy-git situation, but running: ./t7800-difftool.sh --stress --run=2,13 never seems to fail. However, by my reading of reuse_worktree_file(), this would probably always fail under Cygwin, because it sets NO_FAST_WORKING_DIRECTORY. At any rate, since reuse_worktree_file() is meant to be an optimization that may or may not trigger, our test should be robust either way. Instead of checking the filename, let's just make sure we got a single line of output (which would not be true if we continued after the first failure). Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>