[please cc me in replies, as I’m not subscribed to this list]

Before this change, zsh startup time was dominated by lseek(2) system calls on
the history file, as shown by strace -c:

time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 97,35    1,112890           1    697153         1 lseek
  0,99    0,011314           2      5277           read
[…]

This change keeps track of read bytes and the position within the file, removing
all of these system calls.

I verified correctness of the change by comparing fpos with ftell(in) in every
loop iteration.