Add option to switch to external git tool (git.exe) about mbt HOT 8 OPEN

@BuddySpike Yes, I can confirm that now it works as expected.
I still think that having implementation of Repo that will wrap git.exe could be useful, but it could be postponed.

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Thanks for reporting this issue.

1. Long path is definitely going to be an issue. Which version of Windows are you using? I will get back to you on this after checking a couple of things.

2. Are you running it in a Windows container? Which mbt command is used?

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1. Windows 10. Version 1709 with long paths enabled in operation system. And I see the same problem on Windows Server 2016 machine.
   You can see the details and original issue on libgit2 here: libgit2/libgit2#3053
2. Yes, I tried to use mbt from Windows Docker container. Result of the following commands seems to be empty for me:
   mbt describe local --all
mbt describe commit <sha of HEAD>
   After additional experiment it seems that we have this problem only in mounted directories. I will double check it one more time.

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Thanks. Trying to reproduce the container scenario here. What's the base image your reference in your Dockerfile?

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We are creating container from very basic image:
FROM microsoft/windowsservercore
I also trying to perform additional tests and for now it seems that additional condition to reproduce the bug is to have repository directory mounted from host machine docker run --rm -it -v c:\Host\Path:c:\work <image>

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On this end, only mbt describe local --all is returning an empty list. Looking at the root cause at the moment.

In terms of the long path issue, I think it's best to fix it in libgit2. Switching to git cli introduces other issues associated with parsing stdout (in fact this is the main reason why we used libgit2). Having said that, using plumbing commands in git (e.g. git ls-tree | diff-tree etc..) seems like a safe bet as they are built for programatic access.

Implementing this would be a matter of having an implementation of Repo interface (https://github.com/mbtproject/mbt/blob/master/lib/system.go#L68) to for git cli. Then at the time we compose system, we could use that implementation based on a flag (https://github.com/mbtproject/mbt/blob/master/lib/system.go#L422).

Is this something that you could give us a hand with?

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Quick update on this. It looks like an issue in the filepath.Walk API on mounted volumes in Windows. I created a simple bin to test (see below) and it does not work under the conditions mentioned in this bug report.

I want to take a look at Walk implementation and also try to get some assistance from go team (it could very well be an issue with docker/windows as well).

package main

import (
	"flag"
	"fmt"
	"os"
	"path/filepath"
)

func main() {
	path := flag.String("path", "", "path to walk")
	flag.Parse()
	fmt.Printf("walking path %v\n", *path)

	err := filepath.Walk(*path, func(path string, info os.FileInfo, err error) error {
		if err != nil {
			panic(err)
		}
		fmt.Printf("visiting %v\n", path)
		return nil
	})

	if err != nil {
		panic(err)
	}
}

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@sergey-ostapenko I think this is also indirectly fixed by your local discovery optimisation. Could you please check?
(Not long path issue, just the one that didn't report local modules in the container)

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