xgettext: Fix parsing of escaped single-quote characters in shell parser.
Reported by Markus Gothe <nietzsche@lysator.liu.se>
at <https://savannah.gnu.org/bugs/?49809>.
* gettext-tools/src/x-sh.c (phase2_getc): Handle the case c == '\''
&& !open_singlequote && !open_doublequote differently.
* gettext-tools/tests/xgettext-sh-1: Add test with an escape single-quote.
