Fix error detection in executeGitCommand

1. proc.consumeProcessErrorStream does not work correctly: it creates a
   thread to asynchronously copy the content of the stream to the string
   buffer, and does not provide any way to know when it's done. As a
   result, sometimes the buffer is correctly filled in after
   consumeProcessErrorStream returns, sometimes it's not.
2. Checking stderr to know if there was an error is not a good idea: a
   process can run just fine and give output on stderr (logs,
   basically), and a process can fail without giving any output on
   stderr. Checking the status code is more reliable.

release/release.gradle

@@ -1,3 +1,5 @@
+import java.nio.charset.StandardCharsets
+
 import groovy.json.JsonSlurper
 
 /*
@@ -321,13 +323,28 @@ task ciRelease() {
 ciRelease.dependsOn updateChangeLogFile, release
 release.mustRunAfter updateChangeLogFile
 
-void executeGitCommand(String command, String errorMessage){
+static void executeGitCommand(String command, String errorMessage){
     def proc = command.execute()
-    def errorsStringBuffer = new StringBuffer()
-    proc.waitFor()
-    proc.consumeProcessErrorStream( errorsStringBuffer )
-    if ( errorsStringBuffer.toString(  ) != "" ) {
-        throw new GradleException( errorMessage + " " + b );
+    def code = proc.waitFor()
+    def stdout = inputStreamToString( proc.getInputStream() )
+    def stderr = inputStreamToString( proc.getErrorStream() )
+    if ( code != 0 ) {
+        throw new GradleException( errorMessage + "\n\nstdout:\n" + stdout + "\n\nstderr:\n" + stderr )
+    }
+}
+
+static String inputStreamToString(InputStream inputStream) {
+    inputStream.withCloseable { ins ->
+        new BufferedInputStream(ins).withCloseable { bis ->
+            new ByteArrayOutputStream().withCloseable { buf ->
+                int result = bis.read();
+                while (result != -1) {
+                    buf.write((byte) result);
+                    result = bis.read();
+                }
+                return buf.toString( StandardCharsets.UTF_8.name());
+            }
+        }
     }
 }