errors: "strict subset" -> "ancestor"

errors: "strict subset" -> "ancestor"

The term "ancestor" is a bit more intuitive (and more consistent with
the documentation) than the term "strict subset".

Also, remove superfluous "ref", capitalize, and add some carriage
returns, changing:

    error: remote 'refs/heads/master' is not a strict subset of local ref 'refs/heads/master'. maybe you are not up-to-date and need to pull first?
    error: failed to push to 'ssh://linux-nfs.org/~bfields/exports/git.git'

to:

    error: remote 'refs/heads/master' is not an ancestor of
    local 'refs/heads/master'.
    Maybe you are not up-to-date and need to pull first?
    error: failed to push to 'ssh://linux-nfs.org/~bfields/exports/git.git'

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>