checkout: call a single commit "it" intead of "them"

When detached and checking out a branch again, git checkout warns
about commit(s) that might get lost.  It says "If you want to keep
them ..." even for only one commit.

Use Q_() to allow differentiating singular vs plural.

Signed-off-by: Thomas Schneider <thosch97@gmail.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
