Java: Use monotonic timer for comparing timeout instead of system clock. (#15)

Previously the timeout was compared against the system clock, which is
prone to skew and unexpected changes. With this patch the comparison is
made against `System.nanoTime()` which is a high-precision monotically
increasing time designed for elapsed time measurements. This isolates
the timeout from system variation.

Author: jpstotz

CHANGELOG.md

@@ -1,3 +1,10 @@
+# Next version
+
+## Java
+
+ - `Diff_Timeout` is now compared using a monotonically increasing high-resolution
+   timer to isolate the timeout duration from clock skew. (@jpstoz)
+
 # 1.1.0
 
 ## Javascript

java/src/name/fraser/neil/plaintext/diff_match_patch.java

@@ -138,7 +138,7 @@ public LinkedList<Diff> diff_main(String text1, String text2,
     if (Diff_Timeout <= 0) {
       deadline = Long.MAX_VALUE;
     } else {
-      deadline = System.currentTimeMillis() + (long) (Diff_Timeout * 1000);
+      deadline = System.nanoTime() + (long) (Diff_Timeout * 1000000000l);
     }
     return diff_main(text1, text2, checklines, deadline);
   }
@@ -151,7 +151,7 @@ public LinkedList<Diff> diff_main(String text1, String text2,
    * @param checklines Speedup flag.  If false, then don't run a
    *     line-level diff first to identify the changed areas.
    *     If true, then run a faster slightly less optimal diff.
-   * @param deadline Time when the diff should be complete by.  Used
+   * @param deadline Time in nanoseconds when the diff should be complete by.  Used
    *     internally for recursive calls.  Users should set DiffTimeout instead.
    * @return Linked List of Diff objects.
    */
@@ -382,7 +382,7 @@ protected LinkedList<Diff> diff_bisect(String text1, String text2,
     int k2end = 0;
     for (int d = 0; d < max_d; d++) {
       // Bail out if deadline is reached.
-      if (System.currentTimeMillis() > deadline) {
+      if (System.nanoTime() > deadline) {
         break;
       }