fix(path): don't remove trailing slash when getting absolute path

Before Vim patch 8.2.3468 relative_directory is never used in the
resulting path name, so whether it has a trailing slash didn't matter.
Now path_full_dir_name() appends a non-existing relative directory to
the current directory name, so the trailing slash needs to be kept.

src/nvim/path.c

@@ -2362,16 +2362,9 @@ static int path_to_absolute(const char *fname, char *buf, size_t len, int force)
  }
 #endif
  if (p != NULL) {
- // relative to root
- if (p == fname) {
- // only one path component
- relative_directory[0] = PATHSEP;
- relative_directory[1] = NUL;
- } else {
- assert(p >= fname);
- memcpy(relative_directory, fname, (size_t)(p - fname));
- relative_directory[p - fname] = NUL;
- }
+ assert(p >= fname);
+ memcpy(relative_directory, fname, (size_t)(p - fname + 1));
+ relative_directory[p - fname + 1] = NUL;
  end_of_path = p + 1;
  } else {
  relative_directory[0] = NUL;