Create common package for git manipulations

[dangreen]

Create common package for git manipulations (for example @conventional-changelog/git-client).

It will include functionality of git-raw-commits, git-semver-tags and other functionality to farther usage in standard-version.

git-raw-commits and git-semver-tags will use this package inside for backward compatibility, but will deprecated.

[dangreen]

#1203
#1199