Make calls to Git optional

[alexsharoff]

Description

We run swift-package-manager on CI and lately we've noticed that Git's index.lock is left after a build is cancelled. That is a problem, because the lock file prevents post-build repository cleanup from working, i.e. git calls return an error.

Git is called in somewhere here

swift-package-manager/Sources/PackageLoading/ManifestLoader.swift

Lines 859 to 862 in c5d77e5

	gitInformation = ContextModel.GitInformation(
	currentTag: repo.getCurrentTag(),
	currentCommit: try repo.getCurrentRevision().identifier,
	hasUncommittedChanges: repo.hasUncommittedChanges()

Is it possible to disable calling Git in swift-package-manager? For example, via an environment variable.

Expected behavior

No response

Actual behavior

No response

Steps to reproduce

No response

Swift Package Manager version/commit hash

No response

Swift & OS version (output of swift --version && uname -a)

No response