fix(git): Remove Files.isHidden guard in AddExcludedFilesToIndex::prepare

[ytausch]

The goal of prepare() is to prepare a TreeSet toExclude which covers a set of arguments to pass to git add that cover all destination-only files in HEAD, without sweeping in any origin-owned file.

A check in line 73 defensively influences the algorithm to not assume that a git add dir also adds dir/.file, which is indicated in a surrounding comment:

copybara/java/com/google/copybara/git/AddExcludedFilesToIndex.java

Lines 73 to 76 in f4f4f79

	if (Files.isHidden(relative)) {
	// File is not included but 'git add dir' doesn't work for 'dir/.file'.
	addPathAndParents(included, relative.getParent());
	}

Therefore, in a minimal example, where tools is destination-owned and contains tools/.keep and tools/script.sh, we would end up with toExclude = {tools/.keep, tools/script.sh}.

However, git does not behave like indicated and a git add dir also adds any hidden subdirectories or files. Thus, it would be sufficient to run git add tools in the example above and not git add tools/.keep tools/script.sh. The code is overdefensive.

Therefore, this PR removes the isHidden check.

The reason why I am fixing this is that, also, Files.isHidden(relative) is broken on Windows: While on POSIX, isHidden simply checks if the basename starts with a dot, it checks a filesystem attribute on Windows. However, since the file path is relative, this happens relative to the current working directory of the JVM, which is not the checked-out repository. This issue could alternatively be fixed by resolving the file path first, but removing the isHidden check is the correct fix.

Testing

I added a regression test, but I am not sure whether you run the CI on Windows as well. I still think it makes sense to keep.