main.cpp: fix broken build for libgit2 0.21

799e22ea0c3f20 in libgit2 renamed a function, which
breaks our build. To fix that we check the current version
and if the USE_LIBGIT21_API definition was used.

Signed-off-by: Lubomir I. Ivanov <neolit123@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

main.cpp

@@ -42,7 +42,11 @@ int main(int argc, char **argv)
 			files.push_back(a);
 		}
 	}
+#if !LIBGIT2_VER_MAJOR && LIBGIT2_VER_MINOR <= 20 && !defined(USE_LIBGIT21_API)
 	git_threads_init();
+#else
+	git_libgit2_init();
+#endif
 	setup_system_prefs();
 	prefs = default_prefs;
 	fill_profile_color();