Refactor: Speed up git execution by using 'command git'

Since we map 'git' to 'hub', hub is a bit slower.  Using 'git' always
uses the builtin.
Feature: Add compatibility aliases for clipboard and open commands

No matter which clipboard commands your system uses to interact with it,
'pbcopy' and 'pbpaste' should work.

Additionally 'open' should work everywhere.
Feature: Add 'git current-branch' subcommand
Feature: Add 'is_true' boolean helper check
Feature: Add 'is_false' boolean helper check
Feature: Add 'shell_is_cygwin' check
Feature: Add 'shell_is_dumb' to check for dumb terminals
Style: Make whitespace changes
Feature: Add 'git-tracking-branch' subcommand
Feature: Add 'git-remote-url' subcommand
Feature: Add 'git-submodule-remove' subcommand
Feature: Add 'git-submodule-move' subcommand
Feature: Add 'git-short-sha' subcommand
Feature: Add 'git-long-sha' subcommand
Feature: Add 'git-root' subcommand
Feature: Add 'git-activity-calendar' subcommand
Feature: Add 'git-create-tracking-branch' subcommand
Feature: Add 'git-current-tag' subcommand
Feature: Add 'git-github-url' subcommand
Feature: Add 'git-github-browse' subcommand