pkg: rev_store can't handle symlinks

[Alizter]

Some opam repositories such as dune-universe/opam-overlays contain symlinks. This causes the rev_store to fail since the parser cannot understand the output of git-ls. Dune raises

Error: exception Not_found
Raised at Dune_re__Core.exec in file "vendor/re/src/core.ml", line 937,
  characters 20-35
Called from Dune_pkg__Rev_store.File.parse.(fun) in file
  "src/dune_pkg/rev_store.ml", line 227, characters 14-29

in this case which under inspection gives the following line from git-ls as problematic:

160000 commit 0f8ea69ad66f745bd52ecae7189b72f8b6d50da8       -	test/rwo.t/book

Presumably this is due to the fact that book is a symlink in that repo and therefore changes the output of git-ls.

In this particular case the symlink occurs outside of packages/ so the case could be made to ignore it, however it seems sensible after discussing with @Leonidas-from-XIV that symlinks could be supported in the files/ directory of packages. Therefore the rev_store should be modified to handle them correctly.

To reproduce my error above, use the following dune-workspace file:

(lang dune 3.11)

(repository
 (name opam-overlays)
 (source "git+https://github.com/dune-universe/opam-overlays.git"))

(context
 (default
  (name default)
  (repositories opam-overlays)))

[Leonidas-from-XIV]

In particular, this seems to be the case because it is a broken symlink, 0f8ea69ad66f745bd52ecae7189b72f8b6d50da8 doesn't point anywhere. I assume ignoring these should be fine, since there's no data that can be retrieved from this.

[Leonidas-from-XIV]

Turns out it is not a symlink, it is a commit object which… I don't know how this happened there. I wonder how to create those, since it seems very strange that a tree contains something else than a file.

[Leonidas-from-XIV]

So given this turned out not to be about symlinks but commit objects and it is fixed now, I'll close this as done.

Edit: Looks like I forgot to merge, but that's done now so the issue is completed.