grep: use static trans-case table

grep: use static trans-case table

In order to prepare the kwset machinery for a case-insensitive search, we
used to use a static table of 256 elements and filled it every time before
calling kwsalloc().  Because the kwset machinery will never modify this
table, just allocate a single instance globally and fill it at the compile
time.

Signed-off-by: Junio C Hamano <gitster@pobox.com>