Fix IsConflictResolved check for submodule

[goran-w]

Improved IsConflicResolved check, to work correctly for submodule conflicts.

Added an extra (lock-free) call to git-status with --porcelain=v2 which makes it easy to check that:

• The file is still "unmerged".
• The file is a submodule (or not).

If the file is a submodule, we always return false (while it's unmerged) regard the conflict as unresolved (while Unstaged), since we can't use text-diff check on submodules and the conflict is not resolved until it's Staged.

[love-linger]

I'd like to change the modifications as follows

First, use different commands for text-files and submodules

public class IsConflictResolved : Command
{
    public IsConflictResolved(string repo, Models.Change change, bool isSubmodule)
    {
        _change = change;
        _isSubmodule = isSubmodule;

        WorkingDirectory = repo;
        Context = repo;
    }

    public bool Result()
    {
        if (!_isSubmodule)
        {
            var opt = new Models.DiffOption(_change, true);
            Args = $"diff -a --ignore-cr-at-eol --check {opt}";
            return ReadToEnd().IsSuccess;
        }
        
        Args = $"--no-optional-locks status --porcelain=v2 -- \"{_change.Path}\"";
        var rs = ReadToEnd();
        return rs.IsSuccess && rs.StdOut.StartsWith('u');
    }

    private readonly Models.Change _change = null;
    private readonly bool _isSubmodule = false;
}

The modify the constructor of SourceGit.ViewModels.Conflict

var isSubmodule = repo.Submodules.Find(x => x.Path.Equals(change.Path, StringComparison.Ordinal)) != null;
IsResolved = new Commands.IsConflictResolved(repo.FullPath, change, isSubmodule).Result();

[goran-w]

@love-linger I rewrote my commit, the new solution is much simpler (if the file is a submodule entry we just bypass IsCommitResolved and regard the conflict as unresolved - it will be resolved when it's Staged).