commit: refer to commit template as s->fp

commit: refer to commit template as s->fp

Instead of maintaining a local variable for it, use s->fp to keep
track of where the commit message template should be written.

This prepares us to take advantage of the status_printf functions,
which use a struct wt_status instead of a FILE pointer to determine
where to send their output.

Signed-off-by: Jonathan Nieder <redacted>
Signed-off-by: Junio C Hamano <redacted>