Implement merging of index entries.

This allows us to generate a textual diff of conflicting files in
bare repositories by performing a merge on the index followed by
repo.merge_file_from_index on the resulting index entries.

pygit2/decl.h

@@ -657,8 +657,34 @@ typedef struct {
 	git_merge_file_favor_t file_favor;
 } git_merge_options;
 
+typedef struct {
+	unsigned int automergeable;
+	const char *path;
+	unsigned int mode;
+	const char *ptr;
+	size_t len;
+} git_merge_file_result;
+
+typedef enum {
+	GIT_MERGE_FILE_DEFAULT = 0,
+	GIT_MERGE_FILE_STYLE_MERGE = 1,
+	GIT_MERGE_FILE_STYLE_DIFF3 = 2,
+	GIT_MERGE_FILE_SIMPLIFY_ALNUM = 4,
+} git_merge_file_flags_t;
+
+typedef struct {
+	unsigned int version;
+	const char *ancestor_label;
+	const char *our_label;
+	const char *their_label;
+	git_merge_file_favor_t favor;
+	git_merge_file_flags_t flags;
+} git_merge_file_options;
+
 #define GIT_MERGE_OPTIONS_VERSION ...
 
 int git_merge_init_options(git_merge_options *opts,	unsigned int version);
 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);
 int git_merge_trees(git_index **out, git_repository *repo, const git_tree *ancestor_tree, const git_tree *our_tree, const git_tree *their_tree, const git_merge_options *opts);
+int git_merge_file_from_index(git_merge_file_result *out, git_repository *repo, const git_index_entry *ancestor, const git_index_entry *ours, const git_index_entry *theirs, const git_merge_file_options *opts);
+void git_merge_file_result_free(git_merge_file_result *result);

pygit2/repository.py

@@ -526,6 +526,40 @@ class Repository(_Repository):
 
         return opts
 
+    def merge_file_from_index(self, ancestor, ours, theirs):
+        """merge_file_from_index(ancestor, ours, theirs) -> str
+
+        Merge files from index.
+
+        Return a :py:class:`str` object with the merge result
+        containing possible conflicts.
+
+        ancestor
+            The index entry which will be used as a common
+            ancestor.
+        ours
+            The index entry to take as "ours" or base.
+        theirs
+            The index entry which will be merged into "ours"
+        """
+        cmergeresult = ffi.new('git_merge_file_result *')
+
+        cancestor = ancestor._to_c()[0] if ancestor is not None else ffi.NULL
+        cours = ours._to_c()[0] if ours is not None else ffi.NULL
+        ctheirs = theirs._to_c()[0] if theirs is not None else ffi.NULL
+
+        err = C.git_merge_file_from_index(
+                cmergeresult, self._repo,
+                cancestor, cours, ctheirs,
+                ffi.NULL);
+        check_error(err)
+
+        ret = ffi.string(cmergeresult.ptr,
+                cmergeresult.len).decode()
+        C.git_merge_file_result_free(cmergeresult)
+
+        return ret
+
     def merge_commits(self, ours, theirs, favor='normal'):
         """Merge two arbitrary commits
 

test/test_repository.py

@@ -198,6 +198,39 @@ class RepositoryTest(utils.BareRepoTestCase):
         written_sha1 = self.repo.create_blob(data)
         self.assertEqual(hashed_sha1, written_sha1)
 
+    def test_conflicts_in_bare_repository(self):
+        def create_conflict_file(repo, branch, content):
+            oid = repo.create_blob(content)
+            tb = repo.TreeBuilder()
+            tb.insert('conflict', oid, pygit2.GIT_FILEMODE_BLOB)
+            tree = tb.write()
+
+            sig = pygit2.Signature('Author', 'author@example.com')
+            commit = repo.create_commit(branch.name, sig, sig,
+                    'Conflict', tree, [branch.target])
+            self.assertIsNotNone(commit)
+            return commit
+
+        b1 = self.repo.create_branch('b1', self.repo.head.peel())
+        c1 = create_conflict_file(self.repo, b1, 'Conflict 1')
+        b2 = self.repo.create_branch('b2', self.repo.head.peel())
+        c2 = create_conflict_file(self.repo, b2, 'Conflict 2')
+
+        index = self.repo.merge_commits(c1, c2)
+        self.assertIsNotNone(index.conflicts)
+
+        # ConflictCollection does not allow calling len(...) on it directly so
+        # we have to calculate length by iterating over its entries
+        self.assertEqual(sum(1 for _ in index.conflicts), 1)
+
+        (a, t, o) = index.conflicts['conflict']
+        diff = self.repo.merge_file_from_index(a, t, o)
+        self.assertEqual(diff, '''<<<<<<< conflict
+Conflict 1
+=======
+Conflict 2
+>>>>>>> conflict
+''')
 
 class RepositoryTest_II(utils.RepoTestCase):