Add Git.get_revision_sha() with tests.

src/pip/_internal/vcs/git.py

@@ -89,6 +89,29 @@ class Git(VersionControl):
                 show_stdout=False, cwd=temp_dir.path
             )
 
+    def get_revision_sha(self, dest, rev):
+        """
+        Return a commit hash for the given revision if it names a remote
+        branch or tag.  Otherwise, return None.
+
+        Args:
+          dest: the repository directory.
+          rev: the revision name.
+        """
+        # Pass rev to pre-filter the list.
+        output = self.run_command(['show-ref', rev], cwd=dest,
+                                  show_stdout=False, on_returncode='ignore')
+        refs = {}
+        for line in output.strip().splitlines():
+            sha, ref = line.split(None, 1)
+            refs[ref] = sha
+
+        sha = refs.get('refs/remotes/origin/{}'.format(rev))
+        if sha is not None:
+            return sha
+
+        return refs.get('refs/tags/{}'.format(rev))
+
     def check_rev_options(self, dest, rev_options):
         """Check the revision options before checkout to compensate that tags
         and branches may need origin/ as a prefix.

tests/functional/test_vcs_git.py

@@ -0,0 +1,91 @@
+"""
+Contains functional tests of the Git class.
+"""
+
+from pip.utils.temp_dir import TempDirectory
+from pip.vcs.git import Git
+
+
+def get_head_sha(script, dest):
+    """
+    Return the HEAD sha.
+    """
+    result = script.run('git', 'rev-parse', 'HEAD', cwd=dest)
+    sha = result.stdout.strip()
+    return sha
+
+
+def do_commit(script, dest):
+    script.run(
+        'git', 'commit', '-q', '--author', 'pip <pypa-dev@googlegroups.com>',
+        '--allow-empty', '-m', 'test commit', cwd=dest
+    )
+    sha = get_head_sha(script, dest)
+
+    return sha
+
+
+def add_commits(script, dest, count):
+    """
+    Return a list of the commit hashes from oldest to newest.
+    """
+    shas = []
+    for index in range(count):
+        sha = do_commit(script, dest)
+        shas.append(sha)
+
+    return shas
+
+
+def check_rev(repo_dir, rev, expected_sha):
+    git = Git()
+    assert git.get_revision_sha(repo_dir, rev) == expected_sha
+
+
+def test_get_revision_sha(script):
+    with TempDirectory(kind="testing") as temp:
+        repo_dir = temp.path
+        script.run('git', 'init', cwd=repo_dir)
+        shas = add_commits(script, repo_dir, count=6)
+
+        local_branch_sha = shas[0]
+        tag_sha = shas[1]
+        origin_branch_sha = shas[2]
+        upstream_branch_sha = shas[3]
+        ref_sha = shas[4]
+        head_sha = shas[5]
+        assert head_sha == shas[-1]
+
+        script.run(
+            'git', 'branch', 'local-branch', local_branch_sha, cwd=repo_dir
+        )
+        script.run('git', 'tag', 'v1.0', tag_sha, cwd=repo_dir)
+        script.run(
+            'git', 'update-ref', 'refs/remotes/origin/origin-branch',
+            origin_branch_sha, cwd=repo_dir
+        )
+        script.run(
+            'git', 'update-ref', 'refs/remotes/upstream/upstream-branch',
+            upstream_branch_sha, cwd=repo_dir
+        )
+        script.run(
+            'git', 'update-ref', 'refs/generic-ref', ref_sha, cwd=repo_dir
+        )
+
+        # Test two tags pointing to the same sha.
+        script.run('git', 'tag', 'v2.0', tag_sha, cwd=repo_dir)
+        # Test tags sharing the same suffix as another tag, both before and
+        # after alphabetically.
+        script.run('git', 'tag', 'aaa/v1.0', head_sha, cwd=repo_dir)
+        script.run('git', 'tag', 'zzz/v1.0', head_sha, cwd=repo_dir)
+
+        check_rev(repo_dir, 'local-branch', None)
+        check_rev(repo_dir, 'v1.0', tag_sha)
+        check_rev(repo_dir, 'v2.0', tag_sha)
+        check_rev(repo_dir, 'origin-branch', origin_branch_sha)
+        check_rev(repo_dir, 'upstream-branch', None)
+        check_rev(repo_dir, 'generic-ref', None)
+        # Test passing a valid commit hash.
+        check_rev(repo_dir, tag_sha, None)
+        # Test passing a non-existent name.
+        check_rev(repo_dir, 'does-not-exist', None)