Hide master diff link if base branch is same as stockfish master

[linrock]

The master diff link now only shows up if:

• the base branch is not the same as stockfish master
• we don't know if the base branch is the same (old runs)

See #625 (comment)

[ppigazzini]

When rescheduling the fields are blocked, but after pushing another test type button the master's fields are reset to default and writable.
EDIT: sorry, I posted in the wrong PR :(

[ppigazzini]

@linrock for very old tests it doesn't work perfectly see PROD and DEV (updated w/ with this PR)
https://tests.stockfishchess.org/tests/view/515e5f488f0c3e17002f7d64
https://dfts-0.pigazzini.it/tests/view/515e5f488f0c3e17002f7d64

[linrock]

@ppigazzini yep, that's expected behavior for the link to show up there, isn't it? old tests are from before we started checking the master diff server-side so there's no way to know if the base and master branch are different for those. that's why there's no message, and it's safer to leave the link there if someone wants to manually check the diff.

[ppigazzini]

PROD updated, thank you @linrock :)