Migrate to go-git/go-git v5.0.0 (#10735)

vendor/github.com/go-git/go-git/v5/CONTRIBUTING.md

@@ -0,0 +1,46 @@
+# Contributing Guidelines
+
+source{d} go-git project is [Apache 2.0 licensed](LICENSE) and accepts
+contributions via GitHub pull requests.  This document outlines some of the
+conventions on development workflow, commit message formatting, contact points,
+and other resources to make it easier to get your contribution accepted.
+
+## Support Channels
+
+The official support channels, for both users and contributors, are:
+
+- [StackOverflow go-git tag](https://stackoverflow.com/questions/tagged/go-git) for user questions.
+- GitHub [Issues](https://github.com/src-d/go-git/issues)* for bug reports and feature requests.
+
+*Before opening a new issue or submitting a new pull request, it's helpful to
+search the project - it's likely that another user has already reported the
+issue you're facing, or it's a known issue that we're already aware of.
+
+
+## How to Contribute
+
+Pull Requests (PRs) are the main and exclusive way to contribute to the official go-git project.
+In order for a PR to be accepted it needs to pass a list of requirements:
+
+- You should be able to run the same query using `git`. We don't accept features that are not implemented in the official git implementation.
+- The expected behavior must match the [official git implementation](https://github.com/git/git).
+- The actual behavior must be correctly explained with natural language and providing a minimum working example in Go that reproduces it.
+- All PRs must be written in idiomatic Go, formatted according to [gofmt](https://golang.org/cmd/gofmt/), and without any warnings from [go lint](https://github.com/golang/lint) nor [go vet](https://golang.org/cmd/vet/).
+- They should in general include tests, and those shall pass.
+- If the PR is a bug fix, it has to include a suite of unit tests for the new functionality.
+- If the PR is a new feature, it has to come with a suite of unit tests, that tests the new functionality.
+- In any case, all the PRs have to pass the personal evaluation of at least one of the maintainers of go-git.
+
+### Format of the commit message
+
+Every commit message should describe what was changed, under which context and, if applicable, the GitHub issue it relates to:
+
+```
+plumbing: packp, Skip argument validations for unknown capabilities. Fixes #623
+```
+
+The format can be described more formally as follows:
+
+```
+<package>: <subpackage>, <what changed>. [Fixes #<issue-number>]
+```