If any file changed only parse all files if force-update is true

[p8]

Commit 13e9a44 enforced parsing all files if any file is newer than the previous parse, not only updated files.

However, this did not take into account the --force-update option and would parse all files even if --force-update was false.

After this commit, if --force-update is false, only files newer than the previous parse will be parsed.
If --force-update is true (the default), all files will be parsed if any file newer than the previous parse.

[nobu]

This breaks references to labels in unchanged files from a newer file.

[p8]

Maybe I'm missing some context here, but I think currently RDoc works as follows:

1. Parse all files and generate RDoc files for all files if any file is newer than the last parse.
2. Don't parse/generate any files if --no-force-update option is passed and no file is newer than the last parse.

This seems to make the --no-force-update option obsolete because it will only not generate any files if no files changed.

There doesn't seem to be an option to only update a single RDoc generated file. It either generates all files or no files.
So testing a small change in the documentation in a single file will always generate all RDoc files.
For large projects with a lot of files, this could take some time.

With this change, to not break references to labels the --no-force-update option should be skipped.
As --force-update is true by default, it will generate all files when a single file is updated.

Commit 13e9a44 also specifically mentions --force-update:

This commit makes --force-update option enforce to parse
all files if any file is newer than the previous parse, not only
updated files.

But that commit enforces parsing all files with the --no-force-update option as well.

[nobu]

RDoc can detect labels only in parsed files, and to tell if words look like labels are really labels, it needs to parse those files actually.

Consider a project that has two files, foo.rb and bar.rb.

# foo.rb

# Uses Bar
class Foo 
end

# bar.rb

# Used by Foo
class Bar
end

rdoc . parses both files, and report as Files: 2 and Classes: 2.

Now you touch one, foo.rb, and run rdoc --no-force-update, with your patched version.
Then reported as Files: 1 and Classes: 1, as you expected.
But now the updated file doc/Foo.html does not contain a link to Bar.html.
Again touch the other bar.rb and re-generate, in this time doc/Bar.html looses the link too.