filevercmp: don't treat entire filename as suffix.
Sync with gnulib 1ba2b66ea45f9bc43cdc0f6f93efa59157d2b2ba.
(file_prefixlen): When stripping (\.[A-Za-z~][A-Za-z0-9~]*)*$ suffixes,
do not strip the entire file name.
Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
Sync with gnulib 9f48fb992a3d7e96610c4ce8be969cff2d61a01b.
Problems reported by Michael Debertol in <https://bugs.gnu.org/49239>.
Signed-off-by: Andrew Borodin <aborodin@vmail.ru>