t3417: Add test cases for "rebase --whitespace=fix"

t3417: Add test cases for "rebase --whitespace=fix"

The command "git rebase --whitespace=fix HEAD~<N>" is supposed to
only clean up trailing whitespace, and the expectation is that it
cannot fail.

Unfortunately, if one commit adds a blank line at the end of a file
and a subsequent commit adds more non-blank lines after the blank
line, "git apply" (used indirectly by "git rebase") will fail to apply
the patch of the second commit.

Signed-off-by: Björn Gustavsson <bgustavsson@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>