git-rebase--interactive.sh: replace cut with ${v%% *}

git-rebase--interactive.sh: replace cut with ${v%% *}

Some versions of cut do not cope well with lines that do not end in
an LF. In this case, we can completely avoid cut by using the
${var%% *} parameter expansion (suggested by Brandon Casey).

I found this problem when t3404's "avoid unnecessary reset" failed
due to the "rebase -i" not avoiding updating the tested timestamp.

On a Mac OS X 10.4.11 system:

    % printf '%s' 'foo bar' | /usr/bin/cut -d ' ' -f 1
    cut: stdin: Illegal byte sequence

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