simplify replaceFile using relatedTemplate

simplify replaceFile using relatedTemplate

Now that truncateFilePath and relatedTemplate have both been optimised,
may as well use them in replaceFile, rather than the custom hack it
used.

Removed the windows-specific ifdef as well, because on Windows long
filepaths no longer really a problem, since ghc and git-annex use UNC
converted paths.

replaceFile no longer checks fileNameLengthLimit. That took a syscall,
and since we have an existing file, we know filenames of its length are
supported by the filesystem. Assuming that the withOtherTmp directory is
on the same filesystem as the file replaceFile is being called on, which
I believe it is.

Sponsored-by: Leon Schuermann