graphlog: implement --follow-first
log --graph --follow-first FILE cannot be compared with the regular version
because it never worked: --follow-first is not taken in account in
walkchangerevs() fast path and is explicitely bypassed in FILE case in
walkchangerevs() nested iterate() function.
$ hg init
$ touch unknown
$ touch a
$ hg add a
$ hg ci -m "1"
$ touch b
$ hg add b
$ hg ci -m "2"
Should show unknown
$ hg status
? unknown
$ hg revert -r 0 --all
removing b
Should show unknown and b removed
$ hg status
R b
? unknown
Should show a and unknown
$ ls
a
unknown