Fixes #2608 (QuickOpen shows red on first use) and also makes QuickOpen more responsive on
large projects by not immediately invalidating the cached file list.
Reset StringMatcher caches when file list is retrieved.

Now that the file list in Quick Open is more lazily retrieved, we should
clear out the old StringMatcher caches to avoid possibly stale data
sticking around even after the file list has arrived.
Addresses review feedback from @peterflynn.

* Fixes the detection of no results
* removes extra call to matcher.reset()
* adds a test for StringMatcher.reset
Eliminates "red" Quick Open field while waiting for initial index build
addMatchers needs to be in a beforeEach or a test.
Merge pull request #3184 from adobe/dangoor/2608-quickopen-red

Fixes #2608 (Quick Open shows red on first use)