Avoid Gitaly calls when evaluating `exists` conditional

What does this MR do and why?

This MR reduces the number of Gitaly calls it takes to evaluate exists rules.

Currently, for every build with an exists conditional, we issue a Gitaly request, to check if the repository tree satisfies the conditional. We can instead read the filenames from Gitaly only once, since rules evaluate for the same ref.

Related to #472223 (closed)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

1. Create a new project and note its ID

2. Commit this gitlab.yml with 100 jobs or generate one yourself with gen.rb:

   bundle exec ruby gen.rb 100

3. Retrieve bench.rb and run the benchmark for the project ID:

   bin/rails runner bench.rb -- benchmark 296

Local output:

% bin/rails runner bench.rb -- benchmark 298
Warming up --------------------------------------
             enabled     1.000  i/100ms
            disabled     1.000  i/100ms
Calculating -------------------------------------
             enabled      2.319  (± 0.0%) i/s -     12.000  in   5.183742s
            disabled      0.151  (± 0.0%) i/s -      1.000  in   6.613193s

Comparison:
             enabled:        2.3 i/s
            disabled:        0.2 i/s - 15.34x  slower