Git's `--shallow-since` feature can lead to broken checkouts and a fix isn't likely to happen:
https://public-inbox.org/git/CAPSG9dZV2EPpVKkOMcjv5z+NF7rUu=V-ZkZNx47rCv122HsiKg@mail.gmail.com/T/#u

This commit removes `shallow_since` from the canonical reproducible form of `git_repository` unless the attribute is provided explicitly. In cases where Bazel's attempt at fetching with `--depth=1` fail, e.g. if there is no server support for shallow fetches of arbitrary commits, the progress message is now used to inform the user about this potential for fetch time optimizations.

Fixes #10292
Fixes #12857

Closes #17356.

PiperOrigin-RevId: 508569071
Change-Id: I01e6662e38c236d1800d427f66d48a05df818800