if --no-dedup was enabled in a volume which already contained
symlinked duplicate files, renaming/moving folders could fail
this is due to folder contents being moved one file at a time
(which is how symlink breakage is prevented) except the links
are moved assuming the final directory layout, meaning they
may be intermittently broken during the movie
with no-dedup, the symlinks are converted into full files as
each symlink is encountered, but a temporarily broken symlink
would crash the procedure
fix this by giving `_symlink` a new parameter `fsrc`
which is a known valid inode for data copying purposes
* v1.13.8 broke collision resolving for non-identical files;
the correct filename was reserved but not symlinked to
the original file, leaving a zerobyte file instead.
See v1.14.3 github release notes for remediation info
* add sanchecks for early detection of index/fs desync;
saves performance and gives less confusing logs
