The status function takes a &'tree mut DirstateMap<'on_disk> parameter.
'on_disk borrows a read-only byte buffer with the contents of the
.hg/dirstate file. DirstateMap internally uses represents file paths as
std::borrow::Cow<'on_disk, HgPath>, which borrows the byte buffer when
possible and allocates an owned string if not, such as for files added to the
dirstate after it was loaded from disk.
Previously the return type of of status has a 'tree lifetime, meaning it
could borrow all paths from the DirstateMap. With this changeset, that
lifetime is changed to 'on_disk meaning that only paths from the byte buffer
can be borrowed, and paths allocated by DirstateMap must be copied.
Usually most paths are in the byte buffer, and most paths are not part of the
return value of status, so the number of extra copies should be small.
This change will enable status to mutate the DirstateMap after it has
finished constructing its return value. Previously such mutation would be
prevented by possible on-going borrows.