Check validity of patches on opam lint --check-upstream

[kit-ty-kate]

#5400 changed the tool opam uses to apply patches. This break some of the patches present in opam-repository, whose format is corrupted. GNU patch seems to be more lenient with diff format errors, whereas git apply is more strict.

For example:

#=== ERROR while compiling easy-format.1.0.1 ==================================#
These patches didn't apply at /home/opam/.opam/4.14/.opam-switch/build/easy-format.1.0.1:
  - easy-format-make.diff: "/usr/bin/git -c core.autocrlf=false apply --unsafe-paths -p1 /home/opam/.opam/log/processed-patch-8-c2309c" exited with code 128 "error: corrupt patch at line 5"

One tool that might help us check the state of opam-repository more automatically could be opam lint --check-upstream if patch applications were checked.