support git 2.34.0's handling of merge conflict between annexed and non-annexed file

support git 2.34.0's handling of merge conflict between annexed and non-annexed file

This version of git -- or its new default "ort" resolver -- handles such
a conflict by staging two files, one with the original name and the other
named file~ref. Use unmergedSiblingFile when the latter is detected.

(It doesn't do that when the conflict is between a directory and a file
or symlink though, so see previous commit for how that case is handled.)

The sibling file has to be deleted separately, because cleanConflictCruft
may not delete it -- that only handles files that are annex links,
but the sibling file may be the non-annexed file side of the conflict.

The graftin code had assumed that, when the other side of a conclict
is a symlink, the file in the work tree will contain the non-annexed
content that we want it to contain. But that is not the case with the new
git; the file may be the annex link and needs to be replaced with the
content, while the annex link will be written as a -variant file.

(The weird doesDirectoryExist check in graftin turns out to still be
needed, test suite failed when I tried to remove it.)

Test suite passes with new git with ort resolver default. Have not tried it
with old git or other defaults.

Sponsored-by: Noam Kremen on Patreon