Add git hash (from refs/remotes/origin/master) to error reports.

Signed-off-by: Jeffrey Han <itdelatrisu@gmail.com>

src/itdelatrisu/opsu/ErrorHandler.java

@@ -171,6 +171,12 @@ public class ErrorHandler {
 			if (version != null && !version.equals("${pom.version}")) {
 				sb.append("**Version:** ");
 				sb.append(version);
+				String hash = Utils.getGitHash();
+				if (hash != null) {
+					sb.append(" (");
+					sb.append(hash.substring(0, 12));
+					sb.append(')');
+				}
 				sb.append('\n');
 			}
 			String timestamp = props.getProperty("build.date");

src/itdelatrisu/opsu/Utils.java

@@ -32,6 +32,7 @@ import java.io.BufferedInputStream;
 import java.io.BufferedReader;
 import java.io.File;
 import java.io.FileInputStream;
+import java.io.FileReader;
 import java.io.IOException;
 import java.io.InputStream;
 import java.io.InputStreamReader;
@@ -576,4 +577,29 @@ public class Utils {
 	public static boolean parseBoolean(String s) {
 		return (Integer.parseInt(s) == 1);
 	}
+
+	/**
+	 * Returns the git hash of the remote-tracking branch 'origin/master' from the
+	 * most recent update to the working directory (e.g. fetch or successful push).
+	 * @return the 40-character SHA-1 hash, or null if it could not be determined
+	 */
+	public static String getGitHash() {
+		if (isJarRunning())
+			return null;
+		File f = new File(".git/refs/remotes/origin/master");
+		if (!f.isFile())
+			return null;
+		try (BufferedReader in = new BufferedReader(new FileReader(f))) {
+			char[] sha = new char[40];
+			if (in.read(sha, 0, sha.length) < sha.length)
+				return null;
+			for (int i = 0; i < sha.length; i++) {
+				if (Character.digit(sha[i], 16) == -1)
+					return null;
+			}
+			return String.valueOf(sha);
+		} catch (IOException e) {
+			return null;
+		}
+	}
 }