Fix compiler warning (unused-label) in git2r C code

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Rewrite hashfile test case to work on Windows

* tests/blob.R (Hash file test case): writeLines gives LF on
  Unix/Linux, CRLF on Windows, and therefore the test fails. Use
  writeChar and a binary connection instead.

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>