Apply SRP to various git commands #310

[eugenepeh]

part of #310

All the git commands are embedded in the CommandRunner class.

This violates the Single Responsibility Principle as CommandRunner
should only be responsible for the execution, and not the type of
commands.

As a step towards deprecating the use of CommandRunner, let's export
the commands to respective classes and remove the direct dependency on
CommandRunner.

[yong24s]

LGTM, some nits on inconsistency with use of static imports.

[yamidark]

Some comments and nits

[eugenepeh]

@yamidark @yong24s , thanks for the review!

Have fixed accordingly, ready for next iteration.

[yamidark]

Some more comments.
Need to update the DeveloperGuide diagram from GitDownloader -> GitClone (as well as the Git section)

[eugenepeh]

@yamidark thanks for the indepth review again, have made the changes.