Introduce --ignore-whitespace option and corresponding config bool to
ignore whitespace differences while applying patches, akin to the
'patch' program.

'git am', 'git rebase' and the bash git completion are made aware of
this option.
Signed-off-by: NGiuseppe Bilotta <giuseppe.bilotta@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>