Fix FileSnapshot#save(long) and FileSnapshot#save(Instant)

Use the fallback timestamp resolution as already described in the
javadoc of these methods. Using zero file timestamp resolution doesn't
make sense.

Change-Id: Iaad2a0f99c3be3678e94980a0a368181b6aed38c
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/FileSnapshot.java

@@ -44,7 +44,7 @@
 package org.eclipse.jgit.internal.storage.file;
 
 import static org.eclipse.jgit.util.FS.FileStoreAttributes.FALLBACK_FILESTORE_ATTRIBUTES;
-
+import static org.eclipse.jgit.util.FS.FileStoreAttributes.FALLBACK_TIMESTAMP_RESOLUTION;
 import java.io.File;
 import java.io.IOException;
 import java.nio.file.attribute.BasicFileAttributes;
@@ -176,7 +176,7 @@ public class FileSnapshot {
 	public static FileSnapshot save(long modified) {
 		final Instant read = Instant.now();
 		return new FileSnapshot(read, Instant.ofEpochMilli(modified),
-				UNKNOWN_SIZE, Duration.ZERO, MISSING_FILEKEY);
+				UNKNOWN_SIZE, FALLBACK_TIMESTAMP_RESOLUTION, MISSING_FILEKEY);
 	}
 
 	/**
@@ -196,8 +196,8 @@ public class FileSnapshot {
 	 */
 	public static FileSnapshot save(Instant modified) {
 		final Instant read = Instant.now();
-		return new FileSnapshot(read, modified, UNKNOWN_SIZE, Duration.ZERO,
+		return new FileSnapshot(read, modified, UNKNOWN_SIZE,
-				MISSING_FILEKEY);
+				FALLBACK_TIMESTAMP_RESOLUTION, MISSING_FILEKEY);
 	}
 
 	/** Last observed modification time of the path. */

org.eclipse.jgit/src/org/eclipse/jgit/util/FS.java

@@ -213,13 +213,20 @@ public abstract class FS {
 		private static final Duration UNDEFINED_DURATION = Duration
 				.ofNanos(Long.MAX_VALUE);
 
+		/**
+		 * Fallback filesystem timestamp resolution. The worst case timestamp
+		 * resolution on FAT filesystems is 2 seconds.
+		 */
+		public static final Duration FALLBACK_TIMESTAMP_RESOLUTION = Duration
+				.ofMillis(2000);
+
 		/**
 		 * Fallback FileStore attributes used when we can't measure the
 		 * filesystem timestamp resolution. The last modified time granularity
 		 * of FAT filesystems is 2 seconds.
 		 */
 		public static final FileStoreAttributes FALLBACK_FILESTORE_ATTRIBUTES = new FileStoreAttributes(
-				Duration.ofMillis(2000));
+				FALLBACK_TIMESTAMP_RESOLUTION);
 
 		private static final Map<FileStore, FileStoreAttributes> attributeCache = new ConcurrentHashMap<>();