discovery: only calculate closed branches if required
The number of new closed branches is required for printing in error message. So
let's only calculate them if we need to print error about new branches.
Differential Revision: https://phab.mercurial-scm.org/D6314
missing lfs pointer keys: oid, size
ok
unexpected lfs pointer value: version='https://unknown.github.com/spec/v1'
cannot parse git-lfs text: 'version https://git-lfs.github.com/spec/v1\n\noid sha256:4d7a214614ab2935c943f9e0ff69d22eadbb8f32b1258daaa5e2ca24d17e2393\nsize 12345\nx-foo extra-information\n'
unexpected lfs pointer value: oid='ahs256:4d7a214614ab2935c943f9e0ff69d22eadbb8f32b1258daaa5e2ca24d17e2393'
unexpected lfs pointer value: oid='4d7a214614ab2935c943f9e0ff69d22eadbb8f32b1258daaa5e2ca24d17e2393'
unexpected lfs pointer value: size='0x12345'
ok
cannot parse git-lfs text: 'version https://git-lfs.github.com/spec/v1\noid sha256:4d7a214614ab2935c943f9e0ff69d22eadbb8f32b1258daaa5e2ca24d17e2393\nsize 12345\nx-foo extra\ninformation\n'
unexpected lfs pointer key: x_foo
missing lfs pointer keys: oid
missing lfs pointer keys: oid, size