git clone --depth 1 --single-branch only fetches the tip of the
requested branch, so any subsequent git checkout <sha> for a non-tip
commit fails with 'reference is not a tree'. The error was caught and
rethrown as 'shallow clone may not contain this commit', but the clone
arguments were never adjusted.
Drop --depth 1 (and --single-branch when no ref is requested) when the
caller supplies options.sha so the desired commit is present in the
local object store. The ref-only path remains shallow.
Fixes#1589
roboomp