For example, "B2:" or ":B2" were accepted as valid range, with the
missing part set to A1, which resulted in a A1:B2 range.

An omitted sheet specifier always results in a valid relative sheet,
hence that flag is set, so testing flags for non-null before the final
result is available isn't sufficient.

Additionally this revealed a flaw in handling of external references
when loading documents if the original file name differs from the
resulting real file name due to relative paths.

Actually now the displayed file name matches the real file name when
such a document was loaded, where before the original file name was
displayed.

Probably such references didn't even work before if the reference
spanned more than one sheet, which is a rather rare case for external
references.

Change-Id: I38035f2e4b3a11c0280457498cf62b02f882d2a1
(cherry picked from commit 56238d58)
Reviewed-on: https://gerrit.libreoffice.org/18035Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>