Set complete flag in completions

JLine 3 automatically appends a space character to the completion
candidate unless you tell it not to by setting its 'complete' parameter.
This behavior is generally nice because it will automatically complete
something like 'foo/testO<TAB>' to 'foo/testOnly ' which allows the user
to start typing the testname without having to enter space. It does,
however, break scripted completions because it will complete
'scripted wat<TAB>' to 'scripted watch/ '

This commit updates the custom completer to append a " " to the initial
completions and check if there are any additional completions available.
If so, we set the complete flag to true and jline will append a space to
the input when the user presses <TAB> or <ENTER>. Otherwise the old
jline2 behavior where no spaces are ever appended is preserved.

internal/util-complete/src/main/scala/sbt/internal/util/LineReader.scala

@@ -57,11 +57,14 @@ object LineReader {
          * `testOnly testOnly\ com.foo.FooSpec` instead of `testOnly com.foo.FooSpec`.
          */
         if (c.append.nonEmpty) {
-          if (!pl.line().endsWith(" ")) {
-            candidates.add(new Candidate(pl.line().split(" ").last + c.append))
-          } else {
-            candidates.add(new Candidate(c.append))
-          }
+          val comp =
+            if (!pl.line().endsWith(" ")) pl.line().split(" ").last + c.append else c.append
+          // tell jline to append a " " if the completion would be valid with a " " appended
+          // which can be the case for input tasks and some commands. We need to exclude
+          // the empty string and ";" which always seem to be present.
+          val complete = (Parser.completions(parser, comp + " ", 10).get.map(_.display) --
+            Set(";", "")).nonEmpty
+          candidates.add(new Candidate(comp, comp, null, null, null, null, complete))
         }
       }
     }