git: Document resolving references doesn't verify object existence

When passing a reference to `git rev-parse`, it'll resolve that
reference to an object ID. That ID mustn't necessarily be a commit, in
fact not even a tag, but could in fact be anything. The most interesting
edge case is also that "anything" also includes "nothing". As a result,
our use of `ResolveRefish` doesn't actually verify that any given object
ID actually exists, but really only works at the reference-level.

While this behaviour actually makes sense, it certainly can be
unexpected. So let's document this behaviour and show ways to actually
resolve it to a certain object type, which will then also check for
existence.
29 jobs for pks-git-resolve-object-existence in 16 minutes and 32 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Build
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gitlab-org
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gitlab-org
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gitlab-org
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gitlab-org
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gitlab-org
danger-review

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danger-review

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  Test
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bundler-audit-dependency_scanning

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code_navigation

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cover

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gemnasium-dependency_scanning

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gosec-sast

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license_scanning

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lint

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objectinfo_fuzz_test

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praefect_sql_connect

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praefect_sql_test

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proto

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gitlab-org
test: [1.13, v2.26.2, test]

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gitlab-org
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gitlab-org
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gitlab-org
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verify

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