Fix github issue body size

[autoantwort]

In the past I sometimes got errors like

➜  vcpkg git:(test-features) ✗   gh issue create -R microsoft/vcpkg --title "[llvm] Build failure" --body-file /Users/leanderSchulten/git_projekte/vcpkg/vcpkg_installed/vcpkg/issue_body.md

Creating issue in microsoft/vcpkg

GraphQL: Body is too long (maximum is 65536 characters) (createIssue)

With this PR the body size is enforced and dynamically distributed between the logs that are copied into the issue body.

[BillyONeal]

I think this needs tests. append_log and append_logs both look unit testable.

[BillyONeal]

I think this needs tests. append_log and append_logs both look unit testable.

(Note: making testable may require sucking the read_contents out and that's OK!)

[BillyONeal]

This LGTM after the size_t<->int issues are fixed. Thanks for the tests!

[BillyONeal]

Thanks!