feat: new git flags

[caarlos0]

Closes #1321

Description

WIP adding --git-branch, --git-single-branch and --git-recurse-submodules flags, which work as their counterparts on git.

This is a PoC, haven't added tests yet because I'm looking for feedback on the impl first...

Also, this is based on more recent versions of go-git, hence I based it on #1319.

Submitter Checklist

These are the criteria that every PR should meet, please check them off as you
review them:

• Includes unit tests
• Adds integration tests if needed.

Reviewer Notes

• The code flow looks good.
• Unit tests and or integration tests added.

Release Notes

- kaniko adds a new flag `--git-branch` to set which branch/tag to be cloned
- kaniko adds a new flag `--git-single-branch` to clone a single branch
- kaniko adds a new flag `--git-recurse-submodules` to change submodule recursions behavior while cloning

[tejal29]

LGTM!