Refactor git_command code into separate file.
Refactor git_salt module into a separate file.
Add a callback to git clone call.
change config.
Recompile.
Add licenses to new files.
Move constants into a separate file.
Recompile.
Add ifndef/ define to header files.
Recompile.
Merge pull request #3548 from dart-lang/git_salt_refactor

Git salt refactor