Better check for VCS location

Identify the likely root directory to pass to the vcs.get_backend_name instead of simply using dist.location. This is necessary to support version control systems such as git and mercurial where the VCS metadata directory only exists off of the root directory.

This is a fix for pypa#713, where "pip freeze" outputs the wrong value for a repository checked out from git if the package puts files in the "src"  directory.

This change is based on @qwcode's comments at pypa#636 (comment).

pip/__init__.py

@@ -132,9 +132,35 @@ def __init__(self, name, req, editable, comments=()):
     _rev_re = re.compile(r'-r(\d+)$')
     _date_re = re.compile(r'-(20\d\d\d\d\d\d)$')
 
+    @staticmethod
+    def get_expected_vcs_root(dist):
+        """Return the expected location of the VCS root directory.
+
+        This is the directory that would have the .git or .hg subdirectory
+        if the package was installed from git or mercurial"""
+        try:
+            egginfo_lines = dist.get_metadata("SOURCES.txt").split()
+        except KeyError:
+            # No SOURCES.txt in metadata, just return location
+            return os.path.normcase(os.path.abspath(dist.location))
+
+        line_with_egginfo = (x for x in egginfo_lines if ".egg-info" in x).next()
+        path_to_trim_from_location = line_with_egginfo.split(dist.project_name)[0]
+        # Remove trailing slash if it exists
+        if path_to_trim_from_location.endswith('/'):
+            path_to_trim_from_location = path_to_trim_from_location[:-1]
+        rind = dist.location.rfind(path_to_trim_from_location)
+
+        location = dist.location
+        if rind >= 0:
+            location = location[:rind]
+        return os.path.normcase(os.path.abspath(location))
+
+
+
     @classmethod
     def from_dist(cls, dist, dependency_links, find_tags=False):
-        location = os.path.normcase(os.path.abspath(dist.location))
+        location = cls.get_expected_vcs_root(dist)
         comments = []
         from pip.vcs import vcs, get_src_requirement
         if vcs.get_backend_name(location):