Previously, in a pattern like "{TIME:yy} {TIME}",
substituteBackupFilePath() would greedily use the entire string
"yy} {TIME" as the format specifier for the first TIME template, instead
of just "yy". Fix this, by adjusting the regular expression.
This ends up changing the behaviour of a weird corner case that is
covered in the tests, so change the test. I don't think anyone cares
about that case, and I think the current behaviour is better there.
Fixes#10505 (proved by adding a test case very similar to what was
reported there).