Preliminary implementation of setup/refresh functions

Added one function (setup) and an alias (refresh simply calls setup).
These functions give the developer one more way to configure the git
executable path. This also allows the user to interactively adjust the
git executable configured during runtime as these functions dynamically
update the executable path for the entire git module.

git/cmd.py

@@ -17,6 +17,7 @@
 import subprocess
 import sys
 import threading
+from textwrap import dedent
 
 from git.compat import (
     string_types,
@@ -182,16 +183,69 @@ def __setstate__(self, d):
     # Enables debugging of GitPython's git commands
     GIT_PYTHON_TRACE = os.environ.get("GIT_PYTHON_TRACE", False)
 
-    # Provide the full path to the git executable. Otherwise it assumes git is in the path
-    _git_exec_env_var = "GIT_PYTHON_GIT_EXECUTABLE"
-    GIT_PYTHON_GIT_EXECUTABLE = os.environ.get(_git_exec_env_var, git_exec_name)
-
     # If True, a shell will be used when executing git commands.
     # This should only be desirable on Windows, see https://github.com/gitpython-developers/GitPython/pull/126
     # and check `git/test_repo.py:TestRepo.test_untracked_files()` TC for an example where it is required.
     # Override this value using `Git.USE_SHELL = True`
     USE_SHELL = False
 
+    # Provide the full path to the git executable. Otherwise it assumes git is in the path
+    @classmethod
+    def refresh(cls, path=None):
+        """Convenience method for refreshing the git executable path."""
+        cls.setup(path=path)
+
+    @classmethod
+    def setup(cls, path=None):
+        """Convenience method for setting the git executable path."""
+        if path is not None:
+            # use the path the user gave
+            os.environ[cls._git_exec_env_var] = path
+        elif cls._git_exec_env_var in os.environ:
+            # fall back to the environment variable that's already set
+            pass
+        else:
+            # hope that git can be found on the user's $PATH
+            pass
+
+        old_git = cls.GIT_PYTHON_GIT_EXECUTABLE
+        new_git = os.environ.get(cls._git_exec_env_var, cls.git_exec_name)
+        cls.GIT_PYTHON_GIT_EXECUTABLE = new_git
+
+        has_git = False
+        try:
+            cls().version()
+            has_git = True
+        except GitCommandNotFound:
+            pass
+
+        if not has_git:
+            err = dedent("""\
+                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 call git.cmd.setup with the full path.
+                """)
+
+            if old_git is None:
+                # on the first setup (when GIT_PYTHON_GIT_EXECUTABLE is
+                # None) we only warn the user and simply set the default
+                # executable
+                cls.GIT_PYTHON_GIT_EXECUTABLE = cls.git_exec_name
+                print("WARNING: %s" % err)
+            else:
+                # after the first setup (when GIT_PYTHON_GIT_EXECUTABLE
+                # is no longer None) we raise an exception and reset the
+                # GIT_PYTHON_GIT_EXECUTABLE to whatever the value was
+                # previously
+                cls.GIT_PYTHON_GIT_EXECUTABLE = old_name
+                raise GitCommandNotFound("git", err)
+
+    _git_exec_env_var = "GIT_PYTHON_GIT_EXECUTABLE"
+    # immediately set with the default value ("git")
+    GIT_PYTHON_GIT_EXECUTABLE = None
+    # see the setup performed below
+
     @classmethod
     def is_cygwin(cls):
         return is_cygwin_git(cls.GIT_PYTHON_GIT_EXECUTABLE)
@@ -828,13 +882,13 @@ def _call_process(self, method, *args, **kwargs):
             - "command options" to be converted by :meth:`transform_kwargs()`;
             - the `'insert_kwargs_after'` key which its value must match one of ``*args``,
               and any cmd-options will be appended after the matched arg.
-        
+
         Examples::
-        
+
             git.rev_list('master', max_count=10, header=True)
-        
+
         turns into::
-        
+
            git rev-list max-count 10 --header master
 
         :return: Same as ``execute``"""
@@ -970,3 +1024,16 @@ def clear_cache(self):
         self.cat_file_all = None
         self.cat_file_header = None
         return self
+
+
+
+# this is where the git executable is setup
+def setup(path=None):
+    Git.setup(path=path)
+
+
+def refresh(path=None):
+    Git.refresh(path=path)
+
+
+setup()