git apply: option to ignore whitespace differences

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: Giuseppe Bilotta <giuseppe.bilotta@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -674,6 +674,7 @@ _git_am ()
 	--*)
 		__gitcomp "
 			--3way --committer-date-is-author-date --ignore-date
+			--ignore-whitespace --ignore-space-change
 			--interactive --keep --no-utf8 --signoff --utf8
 			--whitespace=
 			"
@@ -695,6 +696,7 @@ _git_apply ()
 			--stat --numstat --summary --check --index
 			--cached --index-info --reverse --reject --unidiff-zero
 			--apply --no-add --exclude=
+			--ignore-whitespace --ignore-space-change
 			--whitespace= --inaccurate-eof --verbose
 			"
 		return
@@ -1536,6 +1538,7 @@ _git_config ()
 	__gitcomp "
 		add.ignore-errors
 		alias.
+		apply.ignorewhitespace
 		apply.whitespace
 		branch.autosetupmerge
 		branch.autosetuprebase