Rename `ignore-git` to `omit-git-hash`

[duckymirror]

As mentioned by @jyn514 on Zulip, the ignore-git option of the bootstrap configuration should be renamed to omit-git-hash to better describe its meaning and distinguish it from submodules = false. It indicates that information about the current commit shouldn't be displayed when e.g. rustdoc --version --verbose is run. This isn't clear from the current name.

[duckymirror]

@rustbot label +T-bootstrap +C-cleanup

[jyn514]

I think it's rare for anyone to set this option manually, but we should update src/bootstrap/CHANGELOG.md at the same time we remove the option just in case.

[Mark-Simulacrum]

Do we have this by default in some profile? Otherwise, I would expect setting this to be common - it's necessary to avoid rebuilds after git commit, I think.

[duckymirror]

The default configurations don't use it, but it defaults to channel == "dev" if it's unset.

@rustbot claim