Don't trim trailing space if it is escaped with backslash

According to [1] backslash can escape trailing space in ignore rules.

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

Bug: 463581
Change-Id: I9cf13f8775cb49f0b6d61cfd3ca3fd6d665fccd8
Signed-off-by: Andrey Loskutov <loskutov@gmx.de>

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

@@ -835,6 +835,12 @@ public class IgnoreRuleSpecialCasesTest {
 		assertMatch("a\\\\b", "a\\b", true);
 	}
 
+	@Test
+	public void testEscapedTrailingSpaces() throws Exception {
+		assertMatch("\\ ", " ", true);
+		assertMatch("a\\ ", "a ", true);
+	}
+
 	@Test
 	public void testNotEscapingBackslash() throws Exception {
 		assertMatch("\\out", "\\out", true);

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

@@ -118,7 +118,7 @@ public class PathMatcher extends AbstractMatcher {
 	public static IMatcher createPathMatcher(String pattern,
 			Character pathSeparator, boolean dirOnly)
 			throws InvalidPatternException {
-		pattern = pattern.trim();
+		pattern = trim(pattern);
 		char slash = Strings.getPathSeparator(pathSeparator);
 		// ignore possible leading and trailing slash
 		int slashIdx = pattern.indexOf(slash, 1);
@@ -127,6 +127,29 @@ public class PathMatcher extends AbstractMatcher {
 		return createNameMatcher0(pattern, pathSeparator, dirOnly);
 	}
 
+	/**
+	 * Trim trailing spaces, unless they are escaped with backslash, see
+	 * https://www.kernel.org/pub/software/scm/git/docs/gitignore.html
+	 *
+	 * @param pattern
+	 *            non null
+	 * @return trimmed pattern
+	 */
+	private static String trim(String pattern) {
+		while (pattern.length() > 0
+				&& pattern.charAt(pattern.length() - 1) == ' ') {
+			if (pattern.length() > 1
+					&& pattern.charAt(pattern.length() - 2) == '\\') {
+				// last space was escaped by backslash: remove backslash and
+				// keep space
+				pattern = pattern.substring(0, pattern.length() - 2) + " "; //$NON-NLS-1$
+				return pattern;
+			}
+			pattern = pattern.substring(0, pattern.length() - 1);
+		}
+		return pattern;
+	}
+
 	private static IMatcher createNameMatcher0(String segment,
 			Character pathSeparator, boolean dirOnly)
 			throws InvalidPatternException {