git-arc: Make git arc patch -r smarter

[markj]

Summary

• Make it work even when git arc isn't run from the root of the repo.
• If the patch fails to apply, let git partially apply the patch and generate rej files for inspection.

While here, remove the return value from apply_rev(), it's never
actually used.

[jhb]

Thanks, this works great for me. Sample output:

> git arc patch -c -r D57785
Applying D57785...
fetch: https://reviews.freebsd.org/D57785.diff: size of remote file is not known
/dev/stdout                                            613  B 9394  Bps    00s
Checking patch sys/kern/link_elf.c...
error: while searching for:
                   &resid, td);
               if (error != 0)
                       goto out;
               bzero(segbase + segs[i]->p_filesz,
                   segs[i]->p_memsz - segs[i]->p_filesz);
       }

       ef->dynamic = (Elf_Dyn *) (mapbase + phdyn->p_vaddr - base_vaddr);

error: patch failed: sys/kern/link_elf.c:1213
Applying patch sys/kern/link_elf.c with 1 reject...
Rejected hunk #1.