libgit2 does not respect GIT_CONFIG_NOSYSTEM, causing test failures

[mlcui-corp]

Currently, the command::test_move::test_move_standalone_create_gc_refs and core::test_gc::test_gc_no_init tests fail on my machine with "Could not find repository main branch" errors.

The root cause is that libgit2, used by git-branchless, does not respect the GIT_CONFIG_NOSYSTEM used in tests. As a result, get_main_branch_name might use the system-wide init.defaultBranch config value, which on my machine is main, not master:

git-branchless/git-branchless-lib/src/core/config.rs

Lines 26 to 32 in 2c1a5b9

	/// The following config values are resolved, in order. The first valid value is returned.
	/// - branchless.core.mainBranch
	/// - (deprecated) branchless.mainBranch
	/// - init.defaultBranch
	/// - finally, default to "master"
	#[instrument]
	pub fn get_main_branch_name(repo: &Repo) -> eyre::Result<String> {

There has been some progress to fixing this upstream, see libgit2/libgit2#6399. I don't think this is too actionable by this project - system configurations could have many incompatible configs, and it's infeasible to unset all of them for tests. For example, setting merge.conflictStyle to diff3 causes test failures in the command::test_navigation::test_navigation_merge test.

[arxanas]

Thanks for investigating!