revert: Add support for git_revert_commit via Repository.revert_commit()

This change adds `Repository.revert_commit()` which wraps around
`git_revert_commit` which will return an `Index` with the appropriate
changes to revert the specified commit.

Fixes #710

README.rst

@@ -68,7 +68,7 @@ Authors
   Peter Dave Hello          Philippe Ombredanne       Ridge Kennedy
   Ross Nicoll               Rui Abreu Ferreira        Sheeo
   Soasme                    Vladimir Rutsky           Yu Jianjian
-  chengyuhang               earl
+  chengyuhang               earl                      Mark Adams
 
 License
 ==============

pygit2/decl.h

@@ -922,3 +922,5 @@ typedef enum {
 
 int git_attr_get(const char **value_out, git_repository *repo, uint32_t flags, const char *path, const char *name);
 git_attr_t git_attr_value(const char *attr);
+
+int git_revert_commit(git_index **out, git_repository *repo, git_commit *revert_commit, git_commit *our_commit, unsigned int mainline, const git_merge_options *merge_options);

pygit2/repository.py

@@ -1001,6 +1001,39 @@ class BaseRepository(_Repository):
         err = C.git_repository_set_ident(self._repo, to_bytes(name), to_bytes(email))
         check_error(err)
 
+    def revert_commit(self, revert_commit, our_commit, mainline=0):
+        """Reverts the given Commit against the given "our" Commit,
+           producing an Index that reflects the result of the revert.
+
+        Arguments
+
+        revert_commit
+            The Commit to revert
+        our_commit
+            The Commit to revert against (eg, HEAD)
+        mainline
+            The parent of the revert Commit, if it is a merge (i.e. 1, 2)
+
+        Returns an Index with the result of the revert.
+        """
+        cindex = ffi.new('git_index **')
+        revert_commit_ptr = ffi.new('git_commit **')
+        our_commit_ptr = ffi.new('git_commit **')
+
+        ffi.buffer(revert_commit_ptr)[:] = revert_commit._pointer[:]
+        ffi.buffer(our_commit_ptr)[:] = our_commit._pointer[:]
+
+        opts = ffi.new('git_merge_options *')
+        err = C.git_merge_init_options(opts, C.GIT_MERGE_OPTIONS_VERSION)
+        check_error(err)
+
+        err = C.git_revert_commit(
+            cindex, self._repo, revert_commit_ptr[0], our_commit_ptr[0], mainline, opts
+        )
+        check_error(err)
+
+        return Index.from_c(self, cindex)
+
 
 class Branches(object):
     def __init__(self, repository, flag=GIT_BRANCH_ALL):

test/test_repository.py

@@ -43,7 +43,7 @@ import six
 
 if six.PY2:
     from urllib import pathname2url
-    
+
 if six.PY3:
     from urllib.request import pathname2url
 
@@ -435,6 +435,28 @@ class RepositoryTest_II(utils.RepoTestCase):
         self.repo.stash_drop()
         self.assertRaises(KeyError, self.repo.stash_pop)
 
+    def test_revert(self):
+        master = self.repo.head.peel()
+        commit_to_revert = self.repo['4ec4389a8068641da2d6578db0419484972284c8']
+        parent = commit_to_revert.parents[0]
+        commit_diff_stats = (
+            parent.tree.diff_to_tree(commit_to_revert.tree).stats
+        )
+
+        revert_index = self.repo.revert_commit(commit_to_revert, master)
+        revert_diff_stats = revert_index.diff_to_tree(master.tree).stats
+
+        self.assertEquals(
+            revert_diff_stats.insertions, commit_diff_stats.deletions
+        )
+        self.assertEquals(
+            revert_diff_stats.deletions, commit_diff_stats.insertions
+        )
+        self.assertEquals(
+            revert_diff_stats.files_changed, commit_diff_stats.files_changed
+        )
+
+
 class RepositorySignatureTest(utils.RepoTestCase):
 
     def test_default_signature(self):
@@ -533,7 +555,7 @@ class CloneRepositoryTest(utils.NoRepoTestCase):
         src_repo_relpath = "./test/data/testrepo.git/"
         repo_path = os.path.join(self._temp_dir, "clone-into")
         url = pathname2url(os.path.realpath(src_repo_relpath))
-        
+
         if url.startswith('///'):
             url = 'file:' + url
         else: