Fixed a compile error in VS2013.
Read the submodule branch option from Git 1.8.2.
Use our strnlen on MacOS for backward compat

Apparently MacOS didn't have strnlen on 10.6 and earlier.  To
avoid having linking problems on older versions, we'll just use
our internal version.
Merge pull request #2026 from libgit2/rb/fix-strnlen-on-old-macos

Use our strnlen on MacOS for backward compat
Merge pull request #2022 from KTXSoftware/development

submodule branch option + little VS2013 fix