But please also note that its value 'project--read-file-absolute'
is also broken. It asks confirmation for re-visiting even absolute
filenames, because currently 'C-x p f' adds abbreviated filenames,
but confirmation checks for absolute.
Does the breakage show itself only when you add a history entry using one
value of project-read-file-name-function and then try to look it up when
using another value of project-read-file-name-function?
If so, that's probably bearable enough for now (people don't really change
this value often). Fixing it will require resolving the abbreviation
situation. And we might as well switch to relative names in project-files
first.