builtin/apply: tighten (dis)similarity index parsing

This was prompted by an incorrect warning issued by clang [1], and a
suggestion by Linus to restrict the range to check for values greater
than INT_MAX since these will give bogus output after casting to int.

In fact the (dis)similarity index is a percentage, so reject values
greater than 100.

[1] http://article.gmane.org/gmane.comp.version-control.git/213857

Signed-off-by: John Keeping <john@keeping.me.uk>