Make DefaultCharset an error

FormatDiff decoded what a failed git command printed to stdout and
stderr in the platform's charset, while the output of one that
succeeds, three lines further down, is read as UTF-8. Both are UTF-8
now.

Author: abashev

buildSrc/src/main/groovy/open-java-format.error-prone-conventions.gradle

@@ -27,6 +27,7 @@ tasks.withType(JavaCompile).configureEach {
         }
         error('CanonicalAnnotationSyntax')
         error('CollectorMutability')
+        error('DefaultCharset')
         error('DirectReturn')
         error('EffectivelyPrivate')
         error('FormatStringConcatenation')

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

@@ -135,8 +135,8 @@ private static String gitCommand(Path dir, String... args) throws IOException, I
         Preconditions.checkState(process.waitFor(30, TimeUnit.SECONDS), "git diff took too long to terminate");
         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()));
+                    + "\n stdout: " + new String(process.getInputStream().readAllBytes(), UTF_8)
+                    + "\n stderr: " + new String(process.getErrorStream().readAllBytes(), UTF_8));
         }
         ByteArrayOutputStream baos = new ByteArrayOutputStream();
         process.getInputStream().transferTo(baos);