Provide missing merge flags (libgit2#615)

This change adds two missing merge flags MergeTreeSkipREUC and MergeTreeNoRecursive.

Author: takuji

merge.go

@@ -126,6 +126,14 @@ const (
 	// continue resolving conflicts.  The merge operation will fail with
 	// GIT_EMERGECONFLICT and no index will be returned.
 	MergeTreeFailOnConflict MergeTreeFlag = C.GIT_MERGE_FAIL_ON_CONFLICT
+	// MergeTreeSkipREUC specifies not to write the REUC extension on the
+	// generated index.
+	MergeTreeSkipREUC MergeTreeFlag = C.GIT_MERGE_SKIP_REUC
+	// MergeTreeNoRecursive specifies not to build a recursive merge base (by
+	// merging the multiple merge bases) if the commits being merged have
+	// multiple merge bases. Instead, the first base is used.
+	// This flag provides a similar merge base to `git-merge-resolve`.
+	MergeTreeNoRecursive MergeTreeFlag = C.GIT_MERGE_NO_RECURSIVE
 )
 
 type MergeOptions struct {