SI-7696 Restore --show-log

Look for the diff line in the transcript, which diffs the log
file and the check file, to pick off the log file name, then
just display the contents.

For --show-diff output
```
\# starting 1 test in run
!! 1 - run/foo.scala                             [output differs]
% diff /home/apm/projects/snytt/test/files/run/foo-run.log /home/apm/projects/snytt/test/files/run/foo.check--- empty
+++ foo-run.log
@@ -1,0 +1,1 @@
+oops
\# 0/1 passed, 1 failed in run
```
--show-log shows
```
\# starting 1 test in run
!! 1 - run/foo.scala                             [output differs]
\##### Log file '/home/apm/projects/snytt/test/files/run/foo-run.log' from failed test #####

oops

\# 0/1 passed, 1 failed in run
```
where comment lines are prefixed by \ for purposes of commit message.

Author: som-snytt

src/main/scala/scala/tools/partest/nest/AbstractRunner.scala

@@ -94,6 +94,7 @@ abstract class AbstractRunner(argstr: String) extends {
     if (optVerbose)  NestUI.setVerbose()
     if (optTerse)    NestUI.setTerse()
     if (optShowDiff) NestUI.setDiffOnFail()
+    if (optShowLog)  NestUI.setLogOnFail()
 
     // Early return on no args, version, or invalid args
     if (optVersion) return echo(versionMsg)

src/main/scala/scala/tools/partest/nest/NestUI.scala

@@ -86,9 +86,23 @@ object NestUI {
     }
     else {
       echo(statusLine(state))
-      if (!state.isOk && isDiffy) {
-        val differ = bold(red("% ")) + "diff "
-        state.transcript find (_ startsWith differ) foreach (echo(_))
+      if (!state.isOk) {
+        if (isDiffy) {
+          val differ = bold(red("% ")) + "diff "
+          state.transcript find (_ startsWith differ) foreach (echo(_))
+        }
+        if (isLogging) {
+          import scala.util.matching.Regex
+          def log(f: File) = {
+            echo(bold(cyan(s"##### Log file '$f' from failed test #####\n")))
+            echo(f.fileContents)
+          }
+          val prompt = bold(red("% "))
+          val differ = raw"(?s)${Regex.quote(prompt)}diff (\S*).*".r
+          state.transcript.collect {
+            case differ(f) => f
+          } foreach (log(_))
+        }
       }
     }
   }
@@ -153,11 +167,13 @@ object NestUI {
   var _debug = false
   var _terse = false
   var _diff  = false
+  var _logging = false
 
   def isVerbose = _verbose
   def isDebug = _debug
   def isTerse = _terse
   def isDiffy = _diff
+  def isLogging = _logging
 
   def setVerbose() {
     _verbose = true
@@ -171,6 +187,9 @@ object NestUI {
   def setDiffOnFail() {
     _diff = true
   }
+  def setLogOnFail() {
+    _logging = true
+  }
   def verbose(msg: String) {
     if (isVerbose)
       System.err.println(msg)

src/main/scala/scala/tools/partest/nest/RunnerSpec.scala

@@ -36,6 +36,7 @@ trait RunnerSpec extends Spec with Meta.StdOpts with Interpolation {
 
   heading("Test output options:")
   val optShowDiff     = "show-diff"    / "show diffs for failed tests"       --?
+  val optShowLog      = "show-log"     / "show log files for failed tests"   --?
   val optVerbose      = "verbose"      / "show verbose progress information" --?
   val optTerse        = "terse"        / "show terse progress information"   --?
   val optDebug        = "debug"        / "enable debugging output"           --?