1.50.4 / 1.51: journal: relative include paths can fail

[simonmichael]

I did find this possibly similar regression, in 1.50.4 and 1.51, caused by recent changes:

$ ls -lRf symlink-to-relative-include.j etc
lrwxr-xr-x@ 1 simon  staff  7 Dec  6 11:07 symlink-to-relative-include.j -> etc/a.j

etc:
total 24
drwxr-xr-x@  5 simon  staff  160 Dec  6 11:07 .
drwxr-xr-x@ 21 simon  staff  672 Dec  6 11:07 ..
-rw-r--r--@  1 simon  staff   15 Dec  6 11:07 foo.j
-rw-r--r--@  1 simon  staff   14 Dec  6 11:07 a.j
$ cat etc/a.j
include foo.j
$ hledger-1.43.2 -f symlink-to-relative-include.j files
/Users/simon/src/hledger/hledger/test/journal/include/symlink-to-relative-include.j
/Users/simon/src/hledger/hledger/test/journal/include/etc/foo.j
$ hledger-1.50.3 -f symlink-to-relative-include.j files
/Users/simon/src/hledger/hledger/test/journal/include/symlink-to-relative-include.j
/Users/simon/src/hledger/hledger/test/journal/include/etc/foo.j
$ hledger-1.50.4 -f symlink-to-relative-include.j files
hledger-1.50.4: Error: /Users/simon/src/hledger/hledger/test/journal/include/symlink-to-relative-include.j:1:1:
  |
1 | include foo.j
  | ^
No files were matched by: foo.j

Originally posted by @simonmichael in #2499