Add Repository.merge_commits()

This allows you to merge arbitrary commits, returning an index, which is
useful when dealing with bare repos in which we want to merge.

pygit2/decl.h

@@ -5,6 +5,7 @@ typedef ... git_push;
 typedef ... git_cred;
 typedef ... git_object;
 typedef ... git_tree;
+typedef ... git_commit;
 typedef ... git_index;
 typedef ... git_diff;
 typedef ... git_index_conflict_iterator;
@@ -274,6 +275,18 @@ typedef struct {
 	const char *new_prefix;
 } git_diff_options;
 
+typedef struct {
+	int (*file_signature)(
+		void **out, const git_diff_file *file,
+		const char *fullpath, void *payload);
+	int (*buffer_signature)(
+		void **out, const git_diff_file *file,
+		const char *buf, size_t buflen, void *payload);
+	void (*free_signature)(void *sig, void *payload);
+	int (*similarity)(int *score, void *siga, void *sigb, void *payload);
+	void *payload;
+} git_diff_similarity_metric;
+
 int git_diff_init_options(git_diff_options *opts, unsigned int version);
 int git_diff_index_to_workdir(git_diff **diff, git_repository *repo, git_index *index, const git_diff_options *opts);
 int git_diff_tree_to_index(git_diff **diff, git_repository *repo, git_tree *old_tree, git_index *index, const git_diff_options *opts);
@@ -578,3 +591,23 @@ const git_blame_hunk *git_blame_get_hunk_byindex(git_blame *blame, uint32_t inde
 const git_blame_hunk *git_blame_get_hunk_byline(git_blame *blame, uint32_t lineno);
 int git_blame_file(git_blame **out, git_repository *repo, const char *path, git_blame_options *options);
 void git_blame_free(git_blame *blame);
+
+/*
+ * Merging
+ */
+
+typedef enum { ... } git_merge_tree_flag_t;
+
+typedef enum { ... } git_merge_file_favor_t;
+
+typedef struct {
+	unsigned int version;
+	git_merge_tree_flag_t flags;
+	unsigned int rename_threshold;
+	unsigned int target_limit;
+	git_diff_similarity_metric *metric;
+	git_merge_file_favor_t file_favor;
+} git_merge_options;
+
+
+int git_merge_commits(git_index **out, git_repository *repo, const git_commit *our_commit, const git_commit *their_commit, const git_merge_options *opts);

pygit2/repository.py

@@ -508,6 +508,45 @@ class Repository(_Repository):
         return Index.from_c(self, cindex)
 
     #
+    # Merging
+    #
+    def merge_commits(self, ours, theirs):
+        """Merge two arbitrary commits
+
+        Arguments:
+
+        ours
+            The commit to take as "ours" or base.
+        theirs
+            The commit which will be merged into "ours"
+
+        Both can be any object which peels to a commit or the id
+        (string or Oid) of an object which peels to a commit.
+
+        Returns an index with the result of the merge
+
+        """
+
+        ours_ptr = ffi.new('git_commit **')
+        theirs_ptr = ffi.new('git_commit **')
+        cindex = ffi.new('git_index **')
+
+        if is_string(ours) or isinstance(ours, Oid):
+            ours = self[ours]
+        if is_string(theirs) or isinstance(theirs, Oid):
+            theirs = self[theirs]
+
+        ours = ours.peel(Commit)
+        theirs = theirs.peel(Commit)
+
+        ffi.buffer(ours_ptr)[:] = ours._pointer[:]
+        ffi.buffer(theirs_ptr)[:] = theirs._pointer[:]
+
+        err = C.git_merge_commits(cindex, self._repo, ours_ptr[0], theirs_ptr[0], ffi.NULL)
+        check_error(err)
+
+        return Index.from_c(self, cindex)
+    #
     # Utility for writing a tree into an archive
     #
     def write_archive(self, treeish, archive, timestamp=None):

test/test_merge.py

@@ -141,3 +141,20 @@ class MergeTestWithConflicts(utils.RepoTestCaseForMerging):
         del idx.conflicts['.gitignore']
         self.assertRaises(KeyError, conflicts.__getitem__, '.gitignore')
         self.assertTrue(idx.conflicts is None)
+
+class MergeCommitsTest(utils.RepoTestCaseForMerging):
+
+    def test_merge_commits(self):
+        branch_head_hex = '03490f16b15a09913edb3a067a3dc67fbb8d41f1'
+        branch_id = self.repo.get(branch_head_hex).id
+
+        merge_index = self.repo.merge_commits(self.repo.head.target, branch_head_hex)
+        self.assertTrue(merge_index.conflicts is None)
+        merge_commits_tree = merge_index.write_tree(self.repo)
+
+        self.repo.merge(branch_id)
+        index = self.repo.index
+        self.assertTrue(index.conflicts is None)
+        merge_tree = index.write_tree()
+
+        self.assertEqual(merge_tree, merge_commits_tree)