fancyopts: add early-options parser compatible with getopt()
The next patch will add a flag for strict parsing of early options, where
we'll have to parse all early options at once instead of processing them
one-by-one by dispatch._earlygetopt(). That's why I decided to hook
fancyopts().
All dispatch._early*opt() functions is planned to be replaced with this
function. But in this stable series, only the strict mode will be handled
by fancyopts.earlygetopt().
#require execbit
$ hg init repo
$ cd repo
$ echo foo > foo
$ chmod 644 foo
$ hg ci -qAm '644'
$ chmod 755 foo
$ hg ci -qAm '755'
reverting to rev 0
$ hg revert -a -r 0
reverting foo
$ hg st
M foo
$ hg diff --git
diff --git a/foo b/foo
old mode 100755
new mode 100644
$ cd ..