bug#16232: [PATCH] grep: make --ignore-case (-i) faster (sometimes 10x) in multibyte locales

[Pádraig Brady]

Cool so it does this transformation:

  sed 's/./[\L&\U&]/g'

Though multi byte case handling has all sorts of edge cases (pardon the pun),
and it may not be always valid to treat each character independently?
For example see some of the tests in:
http://git.sv.gnu.org/gitweb/?p=gnulib.git;a=blob;f=tests/unicase/test-ulc-casecmp.c;hb=HEAD

I wonder might this faster path be restricted to a safer but very common input 
subset of:

(MB_CUR_MAX == 1 || (in_utf8 && *c < 0x80))

Also are the following printfs in the test redundant?

> +data=$(      printf "I:$I $i:i")
> +search_str=$(printf "$i:i I:$I")

nice improvement!
Pádraig.