Allow leading literal '#' and '!' in ignore rules if they are escaped

According to [1] backslash can escape leading special characters '#' and
'!' in ignore rules, so that they are treated literally.

[1] https://www.kernel.org/pub/software/scm/git/docs/gitignore.html

Bug: 463581
Change-Id: I4c02927413a9c63ea5dbf2954877080d902ec1b2
Signed-off-by: Andrey Loskutov <loskutov@gmx.de>

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

@@ -835,6 +835,18 @@ public class IgnoreRuleSpecialCasesTest {
 		assertMatch("a\\\\b", "a\\b", true);
 	}
 
+	@Test
+	public void testEscapedExclamationMark() throws Exception {
+		assertMatch("\\!b!.txt", "!b!.txt", true);
+		assertMatch("a\\!b!.txt", "a\\!b!.txt", true);
+	}
+
+	@Test
+	public void testEscapedHash() throws Exception {
+		assertMatch("\\#b", "#b", true);
+		assertMatch("a\\#", "a\\#", true);
+	}
+
 	@Test
 	public void testEscapedTrailingSpaces() throws Exception {
 		assertMatch("\\ ", " ", true);

org.eclipse.jgit/src/org/eclipse/jgit/ignore/FastIgnoreRule.java

@@ -98,24 +98,31 @@ public class FastIgnoreRule {
 		if (pattern.charAt(0) == '#') {
 			this.matcher = NO_MATCH;
 			dirOnly = false;
-		} else {
-			dirOnly = pattern.charAt(pattern.length() - 1) == PATH_SEPARATOR;
-			if (dirOnly) {
-				pattern = stripTrailing(pattern, PATH_SEPARATOR);
-				if (pattern.length() == 0) {
-					this.matcher = NO_MATCH;
-					return;
-				}
+			return;
+		}
+		if (pattern.charAt(0) == '\\' && pattern.length() > 1) {
+			char next = pattern.charAt(1);
+			if (next == '!' || next == '#') {
+				// remove backslash escaping first special characters
+				pattern = pattern.substring(1);
 			}
-			IMatcher m;
-			try {
-				m = PathMatcher.createPathMatcher(pattern,
-						Character.valueOf(PATH_SEPARATOR), dirOnly);
-			} catch (InvalidPatternException e) {
-				m = NO_MATCH;
+		}
+		dirOnly = pattern.charAt(pattern.length() - 1) == PATH_SEPARATOR;
+		if (dirOnly) {
+			pattern = stripTrailing(pattern, PATH_SEPARATOR);
+			if (pattern.length() == 0) {
+				this.matcher = NO_MATCH;
+				return;
 			}
-			this.matcher = m;
 		}
+		IMatcher m;
+		try {
+			m = PathMatcher.createPathMatcher(pattern,
+					Character.valueOf(PATH_SEPARATOR), dirOnly);
+		} catch (InvalidPatternException e) {
+			m = NO_MATCH;
+		}
+		this.matcher = m;
 	}
 
 	/**