Consider only escaping backslash for regular expressions in ignore rules

While checking if we should consider an ignore rule without '[]'
brackets as a regular expression, check if the backslash escapes one of
the glob special characters '?', '*', '[', '\\'. If not, backslash is
not a part of a regex and should be treated literally.

Bug: 463581
Change-Id: I85208c7f85246fbf6c5029ce3c8b7bb8f4dbd947
Signed-off-by: Andrey Loskutov <loskutov@gmx.de>

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

@@ -835,6 +835,16 @@ public class IgnoreRuleSpecialCasesTest {
 		assertMatch("a\\\\b", "a\\b", true);
 	}
 
+	@Test
+	public void testNotEscapingBackslash() throws Exception {
+		assertMatch("\\out", "\\out", true);
+		assertMatch("\\out", "a/\\out", true);
+		assertMatch("c:\\/", "c:\\/", true);
+		assertMatch("c:\\/", "a/c:\\/", true);
+		assertMatch("c:\\tmp", "c:\\tmp", true);
+		assertMatch("c:\\tmp", "a/c:\\tmp", true);
+	}
+
 	@Test
 	public void testMultipleEscapedCharacters1() throws Exception {
 		assertMatch("\\]a?c\\*\\[d\\?\\]", "]abc*[d?]", true);

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

@@ -142,9 +142,27 @@ public class Strings {
 			if (idx2 > idx1)
 				return true;
 		}
-		// required to match escaped backslashes '\\\\'
-		if (pattern.indexOf('?') != -1 || pattern.indexOf('\\') != -1)
+		if (pattern.indexOf('?') != -1) {
 			return true;
+		} else {
+			// check if the backslash escapes one of the glob special characters
+			// if not, backslash is not part of a regex and treated literally
+			int backSlash = pattern.indexOf('\\');
+			if (backSlash >= 0) {
+				int nextIdx = backSlash + 1;
+				if (pattern.length() == nextIdx) {
+					return false;
+				}
+				char nextChar = pattern.charAt(nextIdx);
+				if (nextChar == '?' || nextChar == '*' || nextChar == '['
+				// required to match escaped backslashes '\\\\'
+						|| nextChar == '\\') {
+					return true;
+				} else {
+					return false;
+				}
+			}
+		}
 		return false;
 	}