Git: set git config diff.color to false

With diff.color=true, which seems the default or frequently used option,
the patch will contains color info. DiffUtils cannot handle this. So,
just turn off this option explicitly.

Change-Id: I8d8fe972655bea4d638cb34b897346c5b11d5abc
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/git/gitclient.cpp

@@ -355,6 +355,8 @@ QStringList GitDiffEditorController::addConfigurationArguments(const QStringList
     QTC_ASSERT(!args.isEmpty(), return args);
 
     QStringList realArgs = {
+        "-c",
+        "diff.color=false",
         args.at(0),
         "-m", // show diff against parents instead of merge commits
         "-M", "-C", // Detect renames and copies