Added t4133, t4134 - tup graph --combine tests.
Fix t4133, t4134 - use existing file checker.
Added t4135 - make sure 'tup graph .' still works.

I think this broke when adding '--dirs', which should only determine whether or not to display dirs, but should still display files within a specific directory regardless of whether or not --dirs is present.
Fix race in t7057.

We are using 'touch foo/out.txt' to get the monitor to chdir into foo/, but since that may be considered a file overwrite if timestamps are different, when the Tupfile now writes to bar/out.txt, it may not delete foo/out.txt since it has been converted to a normal file. Instead we can add the sleep so the file is always converted to a normal file and check that it still exists.
Added t4136, t4137 - tests for graph --ghosts and --env
tup graph: Prune nodes earlier rather than later.

For combining graphs, we don't want to count ghosts if we're not going to show them. We need to have them pruned from the graph before combining.
Fix t4133, t4134 - use 'tup graph .' instead of tup touch
Added t4138 - don't duplicate links in combined nodes.

Otherwise .h files get really wonky.
Update t4135 - check for .gitignore files.