repository: add Repository.revparse_single().

This provides access to libgit2's 'git_revparse_single'.

README.rst

@@ -270,6 +270,12 @@ The interface for RefLogEntry::
     RefLogEntry.oid_old   -- oid of old reference
     RefLogEntry.oid_new   -- oid of new reference
 
+Revision parsing
+================
+
+You can use any of the fancy `<rev>` forms supported by libgit2::
+
+    >>> commit = repo.revparse_single('HEAD^')
 
 Revision walking
 =================

src/pygit2/repository.c

@@ -198,6 +198,30 @@ Repository_getitem(Repository *self, PyObject *value)
     return lookup_object_prefix(self, &oid, len, GIT_OBJ_ANY);
 }
 
+PyObject *
+Repository_revparse_single(Repository *self, PyObject *py_spec)
+{
+    git_object *c_obj;
+    char *c_spec;
+		char *encoding = "ascii";
+    int err;
+
+    /* 1- Get the C revision spec */
+    c_spec = py_str_to_c_str(py_spec, encoding);
+    if (c_spec == NULL)
+        return NULL;
+
+    /* 2- Lookup */
+    err = git_revparse_single(&c_obj, self->repo, c_spec);
+    if (err < 0)  {
+        PyObject *err_obj = Error_set_str(err, c_spec);
+        free(c_spec);
+        return err_obj;
+    }
+
+    return wrap_object(c_obj, self);
+}
+
 git_odb_object *
 Repository_read_raw(git_repository *repo, const git_oid *oid, size_t len)
 {
@@ -792,6 +816,10 @@ PyMethodDef Repository_methods[] = {
       "Return a list with all the references in the repository."},
     {"lookup_reference", (PyCFunction)Repository_lookup_reference, METH_O,
        "Lookup a reference by its name in a repository."},
+    {"revparse_single", (PyCFunction)Repository_revparse_single, METH_O,
+     "Find an object, as specified by a revision string. See "
+     "`man gitrevisions`, or the documentation for `git rev-parse` for "
+     "information on the syntax accepted."},
     {"create_blob", (PyCFunction)Repository_create_blob,
      METH_VARARGS,
      "Create a new blob from memory"},

test/test_repository.py

@@ -40,6 +40,7 @@ from . import utils
 
 
 HEAD_SHA  = '2cdae28389c059815e951d0bb9eed6533f61a46b'
+PARENT_SHA = '5fe808e8953c12735680c257f56600cb0de44b10'  # HEAD^
 A_HEX_SHA = 'af431f20fc541ed6d5afede3e2dc7160f6f01f16'
 A_BIN_SHA = binascii.unhexlify(A_HEX_SHA.encode('ascii'))
 
@@ -127,6 +128,9 @@ class RepositoryTest(utils.BareRepoTestCase):
     def test_get_workdir(self):
         self.assertEqual(self.repo.workdir, None)
 
+    def test_revparse_single(self):
+        parent = self.repo.revparse_single('HEAD^')
+        self.assertEqual(parent.hex, PARENT_SHA)
 
 
 class RepositoryTest_II(utils.RepoTestCase):