It's because there's a leading space before the tab in front of the comment
here:
https://git.wikimedia.org/blob/mediawiki%2Fcore.git/REL1_19/includes%2FXmlT…
That leading space exists in the patch for 1.19.19, so applies fine when
you're applying it to a fresh copy of the 1.19.18 branch. But, as it was
introduced in the 1.19.10 release (
https://git.wikimedia.org/commitdiff/mediawiki%2Fcore.git/926b9e6) and the
patch for the 1.19.10 release didn't include the space, if you used the
patches for all releases since, it won't be there and the new 1.19.19 patch
will fail to apply the first changes to that file without fixing that space
first.
Cheers,
grunny