R1:2b0ad243d179

Use "git ls-remote" to guess if "git fetch" is a no-op

Summary:
Ref T12296. Ref T12392. Currently, when we're observing a remote repository, we periodically run `git fetch ...`.

Instead, periodically run `git ls-remote` (to list refs in the remote) and `git for-each-ref` (to list local refs) and only continue if the two lists are different.

The motivations for this are:

- In T12296, it appears that doing this is //faster// than doing a no-op `git fetch`. This effect seems to reproduce locally in a…