i18n: Rewrite gettext messages start with dash

i18n: Rewrite gettext messages start with dash

Gettext message in a shell script should not start with '-', one
workaround is adding '--' between gettext and the message, like:

    gettext -- "--exec option ..."

But due to a bug in the xgettext extraction, xgettext can not
extract the actual message for this case. Rewriting the message
is a simpler and better solution.

Reported-by: Vincent van Ravesteijn <redacted>
Signed-off-by: Jiang Xin <redacted>
Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
Reviewed-by: Stefano Lattarini <redacted>
Signed-off-by: Junio C Hamano <redacted>