Completion proposal support for complete jline parameter

- to allow completing single argument with multiple tab clicks,
  for example file paths.
- Backport #512
- Fixes #835

spring-shell-autoconfigure/src/main/java/org/springframework/shell/boot/CompleterAutoConfiguration.java

@@ -63,7 +63,7 @@ public class CompleterAutoConfiguration {
 					p.description(),
 					null,
 					null,
-					true)
+					p.complete())
 				)
 				.forEach(candidates::add);
 		}

spring-shell-core/src/main/java/org/springframework/shell/CompletionProposal.java

@@ -49,6 +49,12 @@ public class CompletionProposal {
 	 */
 	private boolean dontQuote = false;
 
+	/**
+	 * Whether the proposal cant be completed further. By setting complete to false then it will not append an space
+	 * making it easier to continue tab completion
+	 */
+	private boolean complete = true;
+
 	public CompletionProposal(String value) {
 		this.value = this.displayText = value;
 	}
@@ -89,6 +95,16 @@ public class CompletionProposal {
 		return this;
 	}
 
+	public CompletionProposal complete(boolean complete) {
+		this.complete = complete;
+		return this;
+	}
+
+	public boolean complete() {
+		return complete;
+	}
+
+
 	public CompletionProposal dontQuote(boolean dontQuote) {
 		this.dontQuote = dontQuote;
 		return this;

spring-shell-standard/src/main/java/org/springframework/shell/standard/FileValueProvider.java

@@ -44,13 +44,17 @@ public class FileValueProvider implements ValueProvider {
 		String input = completionContext.currentWordUpToCursor();
 		int lastSlash = input.lastIndexOf(File.separatorChar);
 		Path dir = lastSlash > -1 ? Paths.get(input.substring(0, lastSlash + 1)) : Paths.get("");
-        String prefix = input.substring(lastSlash + 1, input.length());
+		String prefix = input.substring(lastSlash + 1);
 
 		try {
 			return Files
 				.find(dir, 1, (p, a) -> p.getFileName() != null && p.getFileName().toString().startsWith(prefix),
 					FOLLOW_LINKS)
-                    .map(p -> new CompletionProposal(p.toString()))
+				.map(p -> {
+					boolean directory = Files.isDirectory(p);
+					String value = p.toString() + (directory ? File.separatorChar : "");
+					return new CompletionProposal(value).complete(!directory);
+				})
 				.collect(Collectors.toList());
 		} catch (IOException e) {
 			throw new UncheckedIOException(e);