Default `test` to `allow_reresolve=false` when a manifest is checked in

[IanButterworth]

Repos with checked in manifests are intentionally fixing their deps, and therefore Pkg.test defaulting to allow_reresolve=false would be more in-line with expectations. Otherwise Pkg.test is allowed to deviate, and although it does warn when re-resolving, it's easy to miss such logs, especially when CI ends up green.

To do this requires figuring out if a manifest is checked-in to git. @DilumAluthge pointed out LibGit2 can do that per file
https://libgit2.org/docs/reference/main/ignore/git_ignore_path_is_ignored.html

[DilumAluthge]

I actually think we probably want this instead: https://libgit2.org/docs/reference/main/status/git_status_file.html

[IanButterworth]

And if JULIA_PKG_USE_CLI_GIT=true ether git ls-files or git rev-parse (pointed out by @GunnarFarneback)

[DilumAluthge]

FWIW, IIUC, the current approach in Pkg.jl is to use LibGit2 for all local (non-cloning operations), regardless of the value of JULIA_PKG_USE_CLI_GIT.

And then the value of JULIA_PKG_USE_CLI_GIT is only consulted for operations such as cloning, which are the cases in which CLI Git and LibGit2 have different behavior.

For local behavior, such as checking the status of a file, I think it's easiest to just always use LibGit2.