read-tree -m -u: avoid getting confused by intermediate symlinks.

When switching from a branch with both x86_64/boot/Makefile and
i386/boot/Makefile to another branch that has x86_64/boot as a
symlink pointing at ../i386/boot, the code incorrectly removed
i386/boot/Makefile.

This was because we first removed everything under x86_64/boot
to make room to create a symbolic link x86_64/boot, then removed
x86_64/boot/Makefile which no longer exists but now is pointing
at i386/boot/Makefile, thanks to the symlink we just created.

This fixes it by using the has_symlink_leading_path() function
introduced previously for git-apply in the checkout codepath.
Earlier, "git checkout" was broken in t4122 test due to this
bug, and the test had an extra "git reset --hard" as a
workaround, which is removed because it is not needed anymore.
Signed-off-by: NJunio C Hamano <junkio@cox.net>

t/t4122-apply-symlink-inside.sh

@@ -34,7 +34,6 @@ test_expect_success setup '
 test_expect_success apply '
 
 	git checkout test &&
-	git reset --hard && #### checkout seems to be buggy
 	git diff --exit-code test &&
 	git diff --exit-code --cached test &&
 	git apply --index test.patch

unpack-trees.c

@@ -264,10 +264,12 @@ static int unpack_trees_rec(struct tree_entry_list **posns, int len,
  * directories, in case this unlink is the removal of the
  * last entry in the directory -- empty directories are removed.
  */
-static void unlink_entry(char *name)
+static void unlink_entry(char *name, char *last_symlink)
 {
 	char *cp, *prev;
 
+	if (has_symlink_leading_path(name, last_symlink))
+		return;
 	if (unlink(name))
 		return;
 	prev = NULL;
@@ -291,11 +293,12 @@ static void unlink_entry(char *name)
 
 static struct checkout state;
 static void check_updates(struct cache_entry **src, int nr,
-		struct unpack_trees_options *o)
+			struct unpack_trees_options *o)
 {
 	unsigned short mask = htons(CE_UPDATE);
 	unsigned cnt = 0, total = 0;
 	struct progress progress;
+	char last_symlink[PATH_MAX];
 
 	if (o->update && o->verbose_update) {
 		for (total = cnt = 0; cnt < nr; cnt++) {
@@ -309,6 +312,7 @@ static void check_updates(struct cache_entry **src, int nr,
 		cnt = 0;
 	}
 
+	*last_symlink = '\0';
 	while (nr--) {
 		struct cache_entry *ce = *src++;
 
@@ -317,13 +321,15 @@ static void check_updates(struct cache_entry **src, int nr,
 				display_progress(&progress, ++cnt);
 		if (!ce->ce_mode) {
 			if (o->update)
-				unlink_entry(ce->name);
+				unlink_entry(ce->name, last_symlink);
 			continue;
 		}
 		if (ce->ce_flags & mask) {
 			ce->ce_flags &= ~mask;
-			if (o->update)
+			if (o->update) {
 				checkout_entry(ce, &state, NULL);
+				*last_symlink = '\0';
+			}
 		}
 	}
 	if (total)