add a flag to got_patch to reverse a patch before applying and the -R flag for `got patch'. ok stsp@

as a cheap optimization got-read-patch was sending only the "\ No newline at end of file" lines that follows an addition (a "+" line). To be able to reverse patches in the future got_patch needs to know about all of these lines instead. No functional changes intended. ok stsp@

reminded by and 'looks fine' to stsp@

this allow to apply patches from subdirectories of the work tree root. Prodded by naddy@, ok stsp@.

Saves us from doing a malloc/free call for every item on the list. ok op@

Siphash suggested by jrick as a better alternative to murmurhash for this use case. with small fixes from and ok op@