Fix spotbugs warning on repo cache size updates

Spotbugs correctly reported a race condition where the check for
insertion of a value might report the value is missing then another
thread inserted the value before the current thread performed the
`put`.  Simpler to `put` the value every time the method is called and
then check for the rare case when the value that was put is smaller
than the value that was there perviously.

Repository size cache is used as a hint.  Errors in the size cache may
lead to suboptimal choices temporarily, but they should not lead to
incorrect behavior.

src/main/java/jenkins/plugins/git/GitToolChooser.java

@@ -269,17 +269,15 @@ private boolean setSizeFromInternalCache(String repoURL) {
     /** Cache the estimated repository size for variants of repository URL */
     private void assignSizeToInternalCache(String repoURL, long repoSize) {
         repoURL = convertToCanonicalURL(repoURL);
-        if (repositorySizeCache.containsKey(repoURL)) {
-            long oldSize = repositorySizeCache.get(repoURL);
-            if (oldSize < repoSize) {
-                LOGGER.log(Level.FINE, "Replacing old repo size {0} with new size {1} for repo {2}", new Object[]{oldSize, repoSize, repoURL});
-                repositorySizeCache.put(repoURL, repoSize);
-            } else if (oldSize > repoSize) {
-                LOGGER.log(Level.FINE, "Ignoring new size {1} in favor of old size {0} for repo {2}", new Object[]{oldSize, repoSize, repoURL});
-            }
-        } else {
+        Long oldSize = repositorySizeCache.put(repoURL, repoSize);
+        if (oldSize == null) {
             LOGGER.log(Level.FINE, "Caching repo size {0} for repo {1}", new Object[]{repoSize, repoURL});
-            repositorySizeCache.put(repoURL, repoSize);
+        } else if (oldSize < repoSize) {
+            LOGGER.log(Level.FINE, "Replaced old repo size {0} with new size {1} for repo {2}", new Object[]{oldSize, repoSize, repoURL});
+        } else if (oldSize > repoSize) {
+            /* Put back the larger old repo size and log a warning. This is not harmful but should be quite rare */
+            LOGGER.log(Level.WARNING, "Ignoring new repo size {1} in favor of old size {0} for repo {2}", new Object[]{oldSize, repoSize, repoURL});
+            repositorySizeCache.put(repoURL, oldSize);
         }
     }