Adding setup for git executable

[kenodegard]

Added a convenience function called refresh. This functions accomplishes 2 things.

The first is that now the user can dynamically set the git path AFTER importing GitPython. In the following example git is neither on the user's $PATH nor is $GIT_PYTHON_GIT_EXECUTABLE defined:

>>> import git
WARNING: Bad git executable. The git executable must be specified in one of the following ways:
    (1) be included in your $PATH, or
    (2) be set via $GIT_PYTHON_GIT_EXECUTABLE, or
    (3) explicitly set via git.setup (or git.refresh).

All git commands will error until this is rectified.

>>> git.refresh("/usr/bin/git")
>>> git.Git().version()
'git version 2.11.0 (Apple Git-81)'
>>> 

The second is that this also means that the git executable can be dynamically refreshed at any time by simply calling refresh:

>>> import git
>>> git.Git.GIT_PYTHON_GIT_EXECUTABLE
'git'
>>> git.refresh("/usr/bin/git")
>>> git.Git.GIT_PYTHON_GIT_EXECUTABLE
'/usr/bin/git'

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@2af601d). Click here to learn what that means.
The diff coverage is 68.88%.

@@            Coverage Diff            @@
##             master     #640   +/-   ##
=========================================
  Coverage          ?   92.65%           
=========================================
  Files             ?       61           
  Lines             ?    10029           
  Branches          ?        0           
=========================================
  Hits              ?     9292           
  Misses            ?      737           
  Partials          ?        0

Impacted Files	Coverage Δ
git/test/test_git.py	98.73% <100%> (ø)
git/cmd.py	83.29% <65.85%> (ø)

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[kenodegard]

@Byron what else would you need from me for this to be considered?

[Byron]

Thanks a ton for your contribution, and my apologies for getting to this PR that late!