git-commit-mode.el: avoid extra newlines after existing pseudo-headers

Currently, if you are adding a psuedo-header to a commit that already
has a block of psuedo-headers you get an extra newline after the
existing pseudo-header block and your new pseudo-header. For example,
given the following commit message:

    .mailmap: map different names with the same email address

    Pretty much one year ago (94b410bba864, Jul 12 2013, .mailmap: Map
    email addresses to names) I cleaned up the output of `git shortlog
    -sne` of git.git by writing a .mailmap file fot the git.git project.

    During the year Jens, Kazuki and Trần contributed to git.git using
    different names, but the same email address; unify them.

    Signed-off-by: Stefan Beller <stefanbeller@googlemail.com>
    Signed-off-by: Junio C Hamano <gitster@pobox.com>

If I was importing that into my repository and adding my SOB to it,
currently I would get the following after doing a `git-commit-signoff':

    .mailmap: map different names with the same email address

    Pretty much one year ago (94b410bba864, Jul 12 2013, .mailmap: Map
    email addresses to names) I cleaned up the output of `git shortlog
    -sne` of git.git by writing a .mailmap file fot the git.git project.

    During the year Jens, Kazuki and Trần contributed to git.git using
    different names, but the same email address; unify them.

    Signed-off-by: Stefan Beller <stefanbeller@googlemail.com>
    Signed-off-by: Junio C Hamano <gitster@pobox.com>

    Signed-off-by: Mitchel Humpherys <mitch.special@gmail.com>

After this change, I get:

    .mailmap: map different names with the same email address

    Pretty much one year ago (94b410bba864, Jul 12 2013, .mailmap: Map
    email addresses to names) I cleaned up the output of `git shortlog
    -sne` of git.git by writing a .mailmap file fot the git.git project.

    During the year Jens, Kazuki and Trần contributed to git.git using
    different names, but the same email address; unify them.

    Signed-off-by: Stefan Beller <stefanbeller@googlemail.com>
    Signed-off-by: Junio C Hamano <gitster@pobox.com>
    Signed-off-by: Mitchel Humpherys <mitch.special@gmail.com>

which is consistent with using git-commit's `-s' switch directly.