Don't assume master branch exists when reinstalling editable package …

…from Git (#4450)

* Add failing test

* Don't assume master branch is default

* Don't expect output to stderr

* Use the 'short ref' instead of the 'full ref'

news/4448.bugfix

@@ -0,0 +1,2 @@
+Reinstalling an editable package from Git no longer assumes that the ``master``
+branch exists.

src/pip/_internal/vcs/git.py

@@ -132,7 +132,7 @@ def obtain(self, dest):
             rev_options = [rev]
             rev_display = ' (to %s)' % rev
         else:
-            rev_options = ['origin/master']
+            rev_options = ['origin/HEAD']
             rev_display = ''
         if self.check_destination(dest, url, rev_options, rev_display):
             logger.info(

tests/functional/test_install_vcs.py

@@ -253,3 +253,32 @@ def test_git_works_with_editable_non_origin_repo(script):
     assert "Error when trying to get requirement" in result.stderr
     assert "Could not determine repository location" in result.stdout
     assert "version-pkg==0.1" in result.stdout
+
+
+@pytest.mark.network
+def test_reinstalling_works_with_editible_non_master_branch(script):
+    """
+    Reinstalling an editable installation should not assume that the "master"
+    branch exists. See https://github.com/pypa/pip/issues/4448.
+    """
+    version_pkg_path = _create_test_package(script)
+
+    # Switch the default branch to something other than 'master'
+    script.run('git', 'branch', '-m', 'foobar', cwd=version_pkg_path)
+
+    script.pip(
+        'install', '-e',
+        '%s#egg=version_pkg' %
+        ('git+file://' + version_pkg_path.abspath.replace('\\', '/')),
+    )
+    version = script.run('version_pkg')
+    assert '0.1' in version.stdout
+
+    _change_test_package_version(script, version_pkg_path)
+    script.pip(
+        'install', '-e',
+        '%s#egg=version_pkg' %
+        ('git+file://' + version_pkg_path.abspath.replace('\\', '/')),
+    )
+    version = script.run('version_pkg')
+    assert 'some different version' in version.stdout