[ignore rules] Fix for character group matcher

Currently we fail to properly recognize character group if the pattern
before character group contains opening bracket.

See comment from Sebastien Arod on https://git.eclipse.org/r/56678/

Change-Id: I70d3657a2a328818ea2bdc1409d18ecb3a85825b
Signed-off-by: Andrey Loskutov <loskutov@gmx.de>

org.eclipse.jgit.test/tst/org/eclipse/jgit/ignore/FastIgnoreRuleTest.java

 	@Test
 	public void testSimpleCharClass() {
 		assertMatched("[a]", "a");
+		assertMatched("][a]", "]a");
 		assertMatched("[a]", "a/");
 		assertMatched("[a]", "a/b");
 

org.eclipse.jgit/src/org/eclipse/jgit/ignore/internal/Strings.java

 	private static boolean isComplexWildcard(String pattern) {
 		int idx1 = pattern.indexOf('[');
 		if (idx1 != -1) {
-			int idx2 = pattern.indexOf(']');
+			int idx2 = pattern.indexOf(']', idx1);
 			if (idx2 > idx1)
 				return true;
 		}