Address review comments.

src/pip/_internal/utils/misc.py

@@ -653,7 +653,7 @@ def call_subprocess(
     show_stdout=True,  # type: bool
     cwd=None,  # type: Optional[str]
     on_returncode='raise',  # type: str
-    returncodes=None,  # type: Optional[Iterable[int]]
+    extra_ok_returncodes=None,  # type: Optional[Iterable[int]]
     command_desc=None,  # type: Optional[str]
     extra_environ=None,  # type: Optional[Mapping[str, Any]]
     unset_environ=None,  # type: Optional[Iterable[str]]
@@ -662,13 +662,13 @@ def call_subprocess(
     # type: (...) -> Optional[Text]
     """
     Args:
-      returncodes: an iterable of integer return codes that are acceptable,
-        in addition to 0. Defaults to None, which means [].
+      extra_ok_returncodes: an iterable of integer return codes that are
+        acceptable, in addition to 0. Defaults to None, which means [].
       unset_environ: an iterable of environment variable names to unset
         prior to calling subprocess.Popen().
     """
-    if returncodes is None:
-        returncodes = []
+    if extra_ok_returncodes is None:
+        extra_ok_returncodes = []
     if unset_environ is None:
         unset_environ = []
     # This function's handling of subprocess output is confusing and I
@@ -745,7 +745,7 @@ def call_subprocess(
             spinner.finish("error")
         else:
             spinner.finish("done")
-    if proc.returncode and proc.returncode not in returncodes:
+    if proc.returncode and proc.returncode not in extra_ok_returncodes:
         if on_returncode == 'raise':
             if (logger.getEffectiveLevel() > std_logging.DEBUG and
                     not show_stdout):

src/pip/_internal/vcs/__init__.py

@@ -467,7 +467,7 @@ def run_command(
         show_stdout=True,  # type: bool
         cwd=None,  # type: Optional[str]
         on_returncode='raise',  # type: str
-        returncodes=None,  # type: Optional[Iterable[int]]
+        extra_ok_returncodes=None,  # type: Optional[Iterable[int]]
         command_desc=None,  # type: Optional[str]
         extra_environ=None,  # type: Optional[Mapping[str, Any]]
         spinner=None  # type: Optional[SpinnerInterface]
@@ -482,7 +482,7 @@ def run_command(
         try:
             return call_subprocess(cmd, show_stdout, cwd,
                                    on_returncode=on_returncode,
-                                   returncodes=returncodes,
+                                   extra_ok_returncodes=extra_ok_returncodes,
                                    command_desc=command_desc,
                                    extra_environ=extra_environ,
                                    unset_environ=self.unset_environ,

src/pip/_internal/vcs/git.py

@@ -84,11 +84,13 @@ def get_current_branch(self, location):
         Return the current branch, or None if HEAD isn't at a branch
         (e.g. detached HEAD).
         """
-        # The -q causes the command to exit with status code 1 instead of
-        # 128 if "HEAD" is not a symbolic ref but a detached HEAD.
+        # git-symbolic-ref exits with empty stdout if "HEAD" is a detached
+        # HEAD rather than a symbolic ref.  In addition, the -q causes the
+        # command to exit with status code 1 instead of 128 in this case
+        # and to suppress the message to stderr.
         args = ['symbolic-ref', '-q', 'HEAD']
         output = self.run_command(
-            args, returncodes=(1, ), show_stdout=False, cwd=location,
+            args, extra_ok_returncodes=(1, ), show_stdout=False, cwd=location,
         )
         ref = output.strip()