git fetcher: use resolveRef for getting revision of reference

* Add regression test
* Fix 'no repo' test so it doesn't succeed if the data is still in cache
* Use git_revparse_single inside git-utils instead of reimplementing the same logic.

src/libfetchers/git.cc

@@ -585,7 +585,7 @@ struct GitInputScheme : InputScheme
                         repoInfo.url
                         );
             } else
-                input.attrs.insert_or_assign("rev", Hash::parseAny(chomp(readFile(localRefFile)), HashAlgorithm::SHA1).gitRev());
+                input.attrs.insert_or_assign("rev", repo->resolveRef(ref).gitRev());
 
             // cache dir lock is removed at scope end; we will only use read-only operations on specific revisions in the remainder
         }