[git log] support "extended regexps"

In particular supports the syntax like: '(git|diff)'
Relates to IDEA-111043, although basic regexp was already supported.

plugins/git4idea/src/git4idea/log/GitLogProvider.java

@@ -308,8 +308,9 @@ public class GitLogProvider implements VcsLogProvider {
     }
 
     if (filterCollection.getTextFilter() != null) {
-      String textFilter = StringUtil.escapeBackSlashes(filterCollection.getTextFilter().getText());
+      String textFilter = filterCollection.getTextFilter().getText();
       filterParameters.add(prepareParameter("grep", textFilter));
+      filterParameters.add("--extended-regexp");
     }
 
     filterParameters.add("--regexp-ignore-case"); // affects case sensitivity of any filter (except file filter)