merge: Expose recursion limit merge option (#642)

The `recursion_limit` merge option provided by libgit2 is currently not
exposed and thus inaccessible to Git2Go users. Let's expose it to let
users control creation of recursive merge bases.

(cherry picked from commit 7e726fda6e)

merge.go

@@ -142,6 +142,7 @@ type MergeOptions struct {
 
 	RenameThreshold uint
 	TargetLimit     uint
+	RecursionLimit  uint
 	FileFavor       MergeFileFavor
 
 	//TODO: Diff similarity metric
@@ -153,6 +154,7 @@ func mergeOptionsFromC(opts *C.git_merge_options) MergeOptions {
 		TreeFlags:       MergeTreeFlag(opts.flags),
 		RenameThreshold: uint(opts.rename_threshold),
 		TargetLimit:     uint(opts.target_limit),
+		RecursionLimit:  uint(opts.recursion_limit),
 		FileFavor:       MergeFileFavor(opts.file_favor),
 	}
 }
@@ -179,6 +181,7 @@ func (mo *MergeOptions) toC() *C.git_merge_options {
 		flags:            C.git_merge_flag_t(mo.TreeFlags),
 		rename_threshold: C.uint(mo.RenameThreshold),
 		target_limit:     C.uint(mo.TargetLimit),
+		recursion_limit:  C.uint(mo.RecursionLimit),
 		file_favor:       C.git_merge_file_favor_t(mo.FileFavor),
 	}
 }