[ARM Incremental TypeSpec] Fails if file is deleted

[mikeharder]

If a swagger file is deleted, check "ARM Incremental TypeSpec" will fail, because it calls "git show HEAD", which fails because the file doesn't exist in "HEAD".

Options:

1. If a swagger file has been deleted (or cannot be read from "HEAD" for any reason), set "incremental-typespec=false". These cases should be rare, and it's always safe to "fail false". This is simpler than other options and should be good enough.

2. If a swagger file has been deleted, read the file from "HEAD^" instead. This seems reasonable, because it uses the state of the file before it was delete to determine if the file was tsp-generated. However, the next part of the check for "did spec folder contain TSP before the PR" now gets complicated, because what if the deleted swagger was the only tsp-generated swagger in the spec? This might be more trouble than it's worth.

3. Some other logic we think maps "deleted typespec-generated swagger" to "is incremental change to typespec RP".

Examples

PR: #32148
Workflow: https://github.com/Azure/azure-rest-api-specs/actions/runs/13410138078/job/37458240060#step:3:94

PR: #32727
Workflow: https://github.com/Azure/azure-rest-api-specs/actions/runs/13428893874/job/37516774509#step:3:123

[mikeharder]

@rkmanda: What do you think?