When an UNC-style path is passed into openTempFile, the returned file
starts with that same style of path. Which can cause problems, eg piping
that filename to git failed. So, convert the output filename to be
relative to the input temp directory.
And follow-on changes.
Note that relatedTemplate was changed to operate on a RawFilePath, and
so when it counts the length, it is now the number of bytes, not the
number of code points. This will just make it truncate shorter strings
in some cases, the truncation is still unicode aware.
When not building with the OsPath flag, toOsPath . fromRawFilePath and
fromRawFilePath . fromOsPath do extra conversions back and forth between
String and ByteString. That overhead could be avoided, but that's the
non-optimised build mode, so didn't bother.
Sponsored-by: unqueued
