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

@@ -98,7 +98,15 @@ public class FastIgnoreRule {
 		if (pattern.charAt(0) == '#') {
 			this.matcher = NO_MATCH;
 			dirOnly = false;
-		} else {
+			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);
+			}
+		}
 		dirOnly = pattern.charAt(pattern.length() - 1) == PATH_SEPARATOR;
 		if (dirOnly) {
 			pattern = stripTrailing(pattern, PATH_SEPARATOR);
@@ -116,7 +124,6 @@ public class FastIgnoreRule {
 		}
 		this.matcher = m;
 	}
-	}
 
 	/**
 	 * Returns true if a match was made. <br>