Speed up find-german-comments

Instead of recreating a text_cat sub-process for every string, start a
single text-cat process (with the -s flag, "per line") and reuse that.

Testing on svl/source shows a speed-up of 7.5 x, down from 45.6s to 6.1s!

To avoid blocking on the pipes set autoflush in text-cat, and use
read_line instead of read on the find-german-comments side.

Change-Id: Ic6c00fc3c1e7cbcbbfff65512bf54ad11f900697
Reviewed-on: https://gerrit.libreoffice.org/9226
Reviewed-by: Michael Meeks <michael.meeks@collabora.com>
Tested-by: Michael Meeks <michael.meeks@collabora.com>