Make sure to overwrite files when "reset --hard" detects conflicts

When performing a "reset --hard" a checkout is done. The pathes are
checked for potential checkout conflicts. But in the end for all
remaining conflicts these files are simply deleted from the working
tree. That's not the right strategy to handle checkout conflicts during
"reset --hard". Instead for every conflict we should simply checkout the
merge commit's content.

This is different from native gits behavior which reports errors when
during a "checkout --hard" a file shows up where a folder was expected.

  "warning: unable to unlink d/c.txt: Not a directory"

Why it is like that in native git was asked in
http://permalink.gmane.org/gmane.comp.version-control.git/279482. Unless
it is explained why native git why this error is reported JGit should
overwrite the files.

Bug: 474842
Change-Id: I08e23822a577aaf22120c5137eb169b6bd08447b
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit.test/tst/org/eclipse/jgit/api/ResetCommandTest.java

@@ -156,6 +156,34 @@ public class ResetCommandTest extends RepositoryTestCase {
 		assertEquals(prevHead, db.readOrigHead());
 	}
 
+	@Test
+	public void testHardResetWithConflicts_DoOverWriteUntrackedFile()
+			throws JGitInternalException,
+			AmbiguousObjectException, IOException, GitAPIException {
+		setupRepository();
+		git.rm().setCached(true).addFilepattern("a.txt").call();
+		assertTrue(new File(db.getWorkTree(), "a.txt").exists());
+		git.reset().setMode(ResetType.HARD).setRef(Constants.HEAD)
+				.call();
+		assertTrue(new File(db.getWorkTree(), "a.txt").exists());
+		assertEquals("content", read(new File(db.getWorkTree(), "a.txt")));
+	}
+
+	@Test
+	public void testHardResetWithConflicts_DoDeleteFileFolderConflicts()
+			throws JGitInternalException,
+			AmbiguousObjectException, IOException, GitAPIException {
+		setupRepository();
+		writeTrashFile("d/c.txt", "x");
+		git.add().addFilepattern("d/c.txt").call();
+		FileUtils.delete(new File(db.getWorkTree(), "d"), FileUtils.RECURSIVE);
+		writeTrashFile("d", "y");
+
+		git.reset().setMode(ResetType.HARD).setRef(Constants.HEAD)
+				.call();
+		assertFalse(new File(db.getWorkTree(), "d").exists());
+	}
+
 	@Test
 	public void testResetToNonexistingHEAD() throws JGitInternalException,
 			AmbiguousObjectException, IOException, GitAPIException {

org.eclipse.jgit/src/org/eclipse/jgit/dircache/DirCacheCheckout.java

@@ -354,8 +354,16 @@ public class DirCacheCheckout {
 				// The index entry is missing
 				if (f != null && !FileMode.TREE.equals(f.getEntryFileMode())
 						&& !f.isEntryIgnored()) {
+					if (failOnConflict) {
 						// don't overwrite an untracked and not ignored file
 						conflicts.add(walk.getPathString());
+					} else {
+						// failOnConflict is false. Putting something to conflicts
+						// would mean we delete it. Instead we want the mergeCommit
+						// content to be checked out.
+						update(m.getEntryPathString(), m.getEntryObjectId(),
+								m.getEntryFileMode());
+					}
 				} else
 					update(m.getEntryPathString(), m.getEntryObjectId(),
 						m.getEntryFileMode());
@@ -390,6 +398,9 @@ public class DirCacheCheckout {
 			if (f != null) {
 				// There is a file/folder for that path in the working tree
 				if (walk.isDirectoryFileConflict()) {
+					// We put it in conflicts. Even if failOnConflict is false
+					// this would cause the path to be deleted. Thats exactly what
+					// we want in this situation
 					conflicts.add(walk.getPathString());
 				} else {
 					// No file/folder conflict exists. All entries are files or