Avoids env var warning when path contains $/%; fix gitpython-developers#832

Author: rgkimball

Diff for: git/repo/base.py

@@ -77,6 +77,7 @@ class Repo(object):
     re_whitespace = re.compile(r'\s+')
     re_hexsha_only = re.compile('^[0-9A-Fa-f]{40}$')
     re_hexsha_shortened = re.compile('^[0-9A-Fa-f]{4,40}$')
+    re_envvars = re.compile(r'(\$(\{\s?)?[a-zA-Z_]\w*(\}\s?)?|%\s?[a-zA-Z_]\w*\s?%)')
     re_author_committer_start = re.compile(r'^(author|committer)')
     re_tab_full_line = re.compile(r'^\t(.*)$')