Url Hinting bug fixes

[redmunds]

This is for #6239 to fix hinting for '../' which was exposed by recent file system changes. Added a unit test for this.

---

This also adds site-root relative path support (i.e. path that start with '/'). Added a unit test for this.

---

I also fixed a bug I noticed where list was not filtered for initial list. Recipe:

1. Shutdown and restart Brackets to clear Url Hinting cache
2. Open a .css file in a folder with other files and folders
3. Add this declaration in a rule: background-image: url("");
4. Put cursor between quotes and type a single letter

Results:
Hints list shows all files and folders in the current folder

Expected:
Only files and folders that start with letter typed are listed (which may be none).

Workaround:
Hit Esc key and then Ctrl-space. This works because it uses cached list, which is filtered.

[redmunds]

Note: I originally fixed all of these items with a nice set of atomic commits :) , that I unfortunately checked into the wrong branch :( (pull request #6258). So, here are the original commits which are easier to review:

• Fix "../" paths. Note this issue was exposed (not introduced) by recent file system changes (c63b62a).
• Add support for site-root relative paths. This was brought up on IRC this week, but I don't think an issue was filed. (c626bdc)
• Separate commit to hoist vars (10e745a).
• Add unit tests (ef2bce9)

Reminder: post comments to this new pull request, not in old pull request or commits.

[RaymondLim]

Reviewing ...

[RaymondLim]

You can remove this TODO.

[redmunds]

Done.

[RaymondLim]

Done reviewing. Only one minor issue of a TODO comment.

[redmunds]

Ready for re-review.