krita_xgettext.sh: Fix modification of multiline msgctxt tags

Summary:
For a message like this:

msgctxt ""
"context string"
msgid "original text"

the former code changed the msgctxt and also added

msgctxt "(qtundo-format)"

because it did not find the existing msgctxt on the previously line
w.r.t. msgid.

To fix the problem, we additionally check:

lastLine !~ /^"/

Thanks to Luigi Toscano for noticing and for his patch that does
basically the same.

CCMAIL: luigi.toscano@tiscali.it

Reviewers: rempt, ltoscano

Reviewed By: rempt, ltoscano

Differential Revision: https://phabricator.kde.org/D1203