Improve logging when git diff fails (#1490)

Improve logging when `git diff` fails

Author: kelvinou01

gradle-palantir-java-format/src/main/java/com/palantir/javaformat/gradle/FormatDiff.java

@@ -134,8 +134,11 @@ private static String gitCommand(Path dir, String... args) throws IOException, I
                 new ProcessBuilder().command(args).directory(dir.toFile()).start();
 
         Preconditions.checkState(process.waitFor(30, TimeUnit.SECONDS), "git diff took too long to terminate");
-        Preconditions.checkState(process.exitValue() == 0, "Expected return code of 0");
-
+        if (process.exitValue() != 0) {
+            throw new RuntimeException("Expected return code of 0 but got " + process.exitValue()
+                    + "\n stdout: " + new String(process.getInputStream().readAllBytes())
+                    + "\n stderr: " + new String(process.getErrorStream().readAllBytes()));
+        }
         ByteArrayOutputStream baos = new ByteArrayOutputStream();
         process.getInputStream().transferTo(baos);
         return new String(baos.toByteArray(), UTF_8).trim();