BF: to decide if Git repo, just check if .git exists

.git could be a symbolic link to the corresponding .git/ directory
(e.g. if a submodule with "old fashion" submodule referencing) or
.git file which would have a line with "gitdir: " pointing to the
actual location for that workdir or submodule

Closes airspeed-velocity#516

asv/plugins/git.py

@@ -41,7 +41,7 @@ def __init__(self, url, mirror_path):
     @classmethod
     def is_local_repo(cls, path):
         return os.path.isdir(path) and (
-            os.path.isdir(os.path.join(path, '.git')) or
+            os.path.exists(os.path.join(path, '.git')) or
             os.path.isdir(os.path.join(path, 'objects')))
 
     @classmethod