Added GIT_HASHSIG_ALLOW_SMALL_FILES to allow computing signatures for small files

The implementation of the hashsig API disallow computing a signature on small files containing fewer than a few lines. This new flag disables this behavior.

git_diff_find_similar() sets this flag by default which means that rename / copy detection of small files will now work. This in turn affects the behavior of the git_status and git_blame APIs which will now detect rename of small files assuming the right options are passed.