Recommit.java

import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.Collection;
import java.util.List;
import java.util.Scanner;

public class Recommit
{
	private static Process runAndWait(String cmd) throws InterruptedException, IOException
	{
		Runtime r = Runtime.getRuntime();
		Process p = r.exec(cmd);
		if (p.waitFor()!=0) {
			InputStream cout = p.getInputStream();
			Scanner s = new Scanner(cout).useDelimiter("\\A");
			if (s.hasNext())
				System.out.println(s.next());
			InputStream cerr = p.getErrorStream();
			s = new Scanner(cerr).useDelimiter("\\A");
			if (s.hasNext())
				System.err.println(s.next());
			throw new RuntimeException("Command \""+cmd+"\" failed");
		}
		return p;
	}

	private static String getOutputFor(String cmd) throws IOException, InterruptedException
	{
		Process p = runAndWait(cmd);
		InputStream cout = p.getInputStream();
		Scanner s = new Scanner(cout).useDelimiter("\\A");
		return s.hasNext() ? s.next() : "";
	}

	private static String getLastCommitHash() throws Exception {
		return getOutputFor("git log -1 --format=%H");
	}

	private static List<String> getFilesToCommit() throws IOException, InterruptedException
	{
		String gitOut = getOutputFor("git status --porcelain");
		String[] lines = gitOut.split("[\\r\\n]+");
		List<String> ret = new ArrayList<>();
		for (String file:lines) {
			if (file.startsWith("??"))
				continue;
			ret.add(file.substring(file.lastIndexOf(' ')+1));
		}
		return ret;
	}

	private static void revertCommit() throws IOException, InterruptedException
	{
		runAndWait("git reset --soft HEAD~1");
	}

	private static void runActionForFiles(String action, List<String> files) throws IOException, InterruptedException
	{
		StringBuilder cmd = new StringBuilder("git "+action);
		for (String f:files) {
			cmd.append(' ').append(f);
		}
		runAndWait(cmd.toString());
	}

	private static void commitLike(String commitToImmitate) throws IOException, InterruptedException
	{
		runAndWait("git commit --reuse-message="+commitToImmitate);
	}

	public static void main(String[] args) throws Exception {
		String lastHash = getLastCommitHash();
		revertCommit();
		List<String> changed = getFilesToCommit();
		runActionForFiles("reset HEAD", changed);
		runActionForFiles("add", changed);
		commitLike(lastHash);
	}
}