Remove old symlinks before updating

Unlike blob updates, symlink updates cannot be done "in place"
writing over an old symlink.  This means that in checkout when we
realize that we can safely update a symlink, we still need to
remove the old one before writing the new.

src/checkout.c

@@ -189,6 +189,10 @@ static int checkout_action_common(
 			action = (action & ~CHECKOUT_ACTION__UPDATE_BLOB) |
 				CHECKOUT_ACTION__UPDATE_SUBMODULE;
 
+		/* to "update" a symlink, we must remove the old one first */
+		if (delta->new_file.mode == GIT_FILEMODE_LINK && wd != NULL)
+			action |= CHECKOUT_ACTION__REMOVE;
+
 		notify = GIT_CHECKOUT_NOTIFY_UPDATED;
 	}
 
@@ -764,20 +768,24 @@ cleanup:
 }
 
 static int blob_content_to_link(
-	struct stat *st, git_blob *blob, const char *path, mode_t dir_mode, int can_symlink)
+	struct stat *st,
+	git_blob *blob,
+	const char *path,
+	mode_t dir_mode,
+	int can_symlink)
 {
 	git_buf linktarget = GIT_BUF_INIT;
 	int error;
 
 	if ((error = git_futils_mkpath2file(path, dir_mode)) < 0)
 		return error;
-	
+
 	if ((error = git_blob__getbuf(&linktarget, blob)) < 0)
 		return error;
 
 	if (can_symlink) {
 		if ((error = p_symlink(git_buf_cstr(&linktarget), path)) < 0)
-			giterr_set(GITERR_CHECKOUT, "Could not create symlink %s\n", path);
+			giterr_set(GITERR_OS, "Could not create symlink %s\n", path);
 	} else {
 		error = git_futils_fake_symlink(git_buf_cstr(&linktarget), path);
 	}