fix(repo-server): excess git requests, short-circuit GenerateManifests ref only (Issue #14725)

[nromriell]

Partially Fixes argoproj#14725

Second part of a split of the argoproj#16309 and follow-up to argoproj#16410. Continues the fixes required to remove the excess git requests when using multi-source and ref sources.

Issue:
When GenerateManifest is called with a ref only source manifest generation is still run, and since there are no manifests this is always considered a manifest cache miss causing an excess number of fetch requests to the git server

Changes:
Short-circuit calls to GenerateManifest when it is a ref only source, however as part of the short circuit still resolve the references if they are not already in cache, this can help prevent additional calls to ls-remote when GenerateManifest is called with a related manifest generating source

Impact:
This PR should greatly reduce the number of fetch calls made to git for multi-source applications with a ref only source. Some minor improvements to ls-remote should be observed as well since we skip unnecessary git util calls.

There is one additional item that will be included in a follow-up PR to finish up the reduction of ls-remote calls

Baseline v2.9.0 with 200 multi-source applications with a ref only source:

With the changes in the first PR under the same conditions:

With the changes from the first PR and this PR under the same conditions:

Checklist:

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[ishitasequeira]

@nromriell Do you mind rebasing the PR with argo-cd/master to be able to get the PR reviewed?