SI-6507 completely sidestep handlers in REPL when :silent in on

This is a cleanup of 6db8a52, the original fix for SI-6507.

When the REPL is :silent, all handlers are ignored when it comes to
generating the printed result. The result extraction code (`lazy val
resN = ...`) is still generated, but now it isn't called until the
user calls it.
Overhaul of partest-ack.

Seems like many/most options to partest are broken. Working
around as much as I can to make this thing useful again, plus
lots of improvements in robustness. Spaces and such are
preserved properly now, and I added a new way of selecting
tests based on time.

  tools/partest-ack -s 1.week.ago
  % tests modified since time       ...  66

  tools/partest-ack 'case object (Foo|Bar)'
  % tests with matching paths       ...  0
  % tests with matching code        ...  7

File paths and file contents are uniformly acked now, so
for instance this works as expected.

  tools/partest-ack -i VIRTPAT
  % tests with matching paths       ...  41
  % tests with matching code        ...  11
  # 43 tests to run.
Merge pull request #2873 from paulp/pr/partest-ack

Overhaul of partest-ack.
Merge pull request #2850 from gourlaysama/wip/t6507-2

SI-6507 completely sidestep handlers in REPL when :silent in on
SI-7781 REPL stack trunc shows cause

The handy stack trace truncation in REPL doesn't
show cause like a regular trace.

This commit fixes that and also adds the usual
indicator for truncation, viz, "... 33 more".

The example from the ticket produces:

```
scala> rewrapperer
java.lang.RuntimeException: java.lang.RuntimeException: java.lang.RuntimeException: Point of failure
  at .rewrapper(<console>:9)
  at .rewrapperer(<console>:10)
  ... 32 elided
Caused by: java.lang.RuntimeException: java.lang.RuntimeException: Point of failure
  at .wrapper(<console>:8)
  ... 34 more
Caused by: java.lang.RuntimeException: Point of failure
  at .sample(<console>:7)
  ... 35 more
```

Suppressed exceptions on Java 7 are handled reflectively.

```
java.lang.RuntimeException: My problem
  at scala.tools.nsc.util.StackTraceTest.repressed(StackTraceTest.scala:56)
  ... 27 elided
  Suppressed: java.lang.RuntimeException: Point of failure
    at scala.tools.nsc.util.StackTraceTest.sample(StackTraceTest.scala:29)
    at scala.tools.nsc.util.StackTraceTest.repressed(StackTraceTest.scala:54)
    ... 27 more
```
Target junit.clean to clean junit artifacts

And all.clean will also junit.clean.