improve how new branches are created

[krancour]

As a safeguard, Kargo render has long refused to write rendered config to a branch that doesn't already appear to be managed via Kargo Render. The signal that a branch is managed by Kargo Render has always been the existence of branch metadata in .kargo-render/metadata.yaml.

This means that when creating new, orphaned, stage-specific branches, we have always written blank metadata to the new branch.

The trouble with this approach is that Kargo proper (rather than Kargo Render) will, at times, be responsible for creation of stage-specific branches -- and when doing so, it doesn't (and shouldn't) particularly care about writing Kargo Render metadata into that new branch.

This PR changes some internal Kargo Render logic so that if we check out an existing branch that doesn't already appear to be managed via Kargo Render, we won't refuse to write to it as long as it's 100% empty. The corollary to this is that when Kargo Render creates new branches, it no longer needs to write blank metadata to them.

So, overall, this is actually quite a simplification.

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[krancour]

I intend to cut this release immediately after this PR is merged.

[codecov]

Codecov Report

Attention: Patch coverage is 18.18182% with 27 lines in your changes are missing coverage. Please review.

Project coverage is 27.57%. Comparing base (bbd4546) to head (fb2228d).

Files	Patch %	Lines
service.go	0.00%	17 Missing ⚠️
branches.go	0.00%	6 Missing ⚠️
pkg/git/git.go	60.00%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #266      +/-   ##
==========================================
- Coverage   27.60%   27.57%   -0.04%     
==========================================
  Files          22       22              
  Lines        2079     2096      +17     
==========================================
+ Hits          574      578       +4     
- Misses       1428     1440      +12     
- Partials       77       78       +1     

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