Use logger's built-in string formatting rather than concatenation

Change-Id: I4dc397fd418f1237cc3cc6194565ab3204c1acb9

java/com/google/gitiles/BlobSoyData.java

diff --git a/java/com/google/gitiles/BlobSoyData.java b/java/com/google/gitiles/BlobSoyData.java
index 2051d2a..956222f 100644
--- a/java/com/google/gitiles/BlobSoyData.java
+++ b/java/com/google/gitiles/BlobSoyData.java
@@ -136,7 +136,7 @@
     } catch (StackOverflowError e) {
       // TODO(dborowitz): Aaagh. Make prettify use RE2. Or replace it something
       // else. Or something.
-      log.warn("StackOverflowError prettifying " + view.toUrl());
+      log.warn("StackOverflowError prettifying {}", view.toUrl());
       return ImmutableList.of(
           new ParseResult(0, content.length(), ImmutableList.of(Prettify.PR_PLAIN)));
     }

java/com/google/gitiles/Linkifier.java

diff --git a/java/com/google/gitiles/Linkifier.java b/java/com/google/gitiles/Linkifier.java
index 493e1c5..c997b8a 100644
--- a/java/com/google/gitiles/Linkifier.java
+++ b/java/com/google/gitiles/Linkifier.java
@@ -83,7 +83,7 @@
         if (log.isDebugEnabled()) {
           log.debug(msg, ex);
         } else {
-          log.warn(msg + ": " + ex.getMessage());
+          log.warn("{}: {}", msg, ex.getMessage());
         }
         continue;
       }