Allow diff_patch to run without the global lock

We noticed that our service would pause and refuse to service any other
requests when a large commit was requested as a patch. This allows
concurrent requests to be processed.

Author: stanhu

ext/rugged/rugged_diff.c

@@ -6,6 +6,7 @@
  */
 
 #include "rugged.h"
+#include <ruby/thread.h>
 
 extern VALUE rb_mRugged;
 VALUE rb_cRuggedDiff;
@@ -197,6 +198,20 @@ static int diff_print_cb(
 	return GIT_OK;
 }
 
+struct nogvl_diff_patch_args {
+	git_diff * diff;
+	git_diff_format_t format;
+	VALUE rb_str;
+};
+
+static void rb_git_diff_patch_nogvl(void * _args)
+{
+	struct nogvl_diff_patch_args * args;
+
+	args = (struct nogvl_diff_patch_args *)_args;
+	git_diff_print(args->diff, args->format, diff_print_cb, (void*) args->rb_str);
+}
+
 /*
  *  call-seq:
  *    diff.patch -> patch
@@ -209,20 +224,25 @@ static VALUE rb_git_diff_patch(int argc, VALUE *argv, VALUE self)
 	git_diff *diff;
 	VALUE rb_str = rb_str_new(NULL, 0);
 	VALUE rb_opts;
+	struct nogvl_diff_patch_args args;
+	git_diff_format_t format = GIT_DIFF_FORMAT_PATCH;
 
 	rb_scan_args(argc, argv, "00:", &rb_opts);
 
 	Data_Get_Struct(self, git_diff, diff);
 
 	if (!NIL_P(rb_opts)) {
 		if (rb_hash_aref(rb_opts, CSTR2SYM("compact")) == Qtrue)
-			git_diff_print(diff, GIT_DIFF_FORMAT_NAME_STATUS, diff_print_cb, (void*)rb_str);
+			format = GIT_DIFF_FORMAT_NAME_STATUS;
 		else
-			git_diff_print(diff, GIT_DIFF_FORMAT_PATCH, diff_print_cb, (void*)rb_str);
-	} else {
-		git_diff_print(diff, GIT_DIFF_FORMAT_PATCH, diff_print_cb, (void*)rb_str);
+			format = GIT_DIFF_FORMAT_PATCH;
 	}
 
+	args.diff = diff;
+	args.format = format;
+	args.rb_str = rb_str;
+	rb_thread_call_without_gvl(rb_git_diff_patch_nogvl, &args, RUBY_UBF_PROCESS, NULL);
+
 	return rb_str;
 }