Parse and apply all the unified patch files from opam-repository

[kit-ty-kate]

In its current state, this PR fixes all parsing error. However some patch files are simply ignored as they use different formats (e.g. contextual diff, which can unfortunately be found in opam-repository)

This will be a draft until i manage to ensure no difference between GNU patch and this version of patch for every packages in opam-repository (this will also require #18)

[hannesm]

n8ce. would be great to add these patch files as regression tests to the testsuite here (also the contextual diffs).

[kit-ty-kate]

Week update: using ocaml/opam#5892 and this PR, only 9 out of the 915 packages with patches differ from GNU patch.
The remaining diff is:

Files opam-2.3.0-alpha1/dbm.1.0/Makefile and opam+ocaml-patch/dbm.1.0/Makefile differ
Only in opam+ocaml-patch/frama-c.22.0/tests/fc_script/oracle: flamegraph.err
Only in opam+ocaml-patch/frama-c.22.0/tests/fc_script/oracle: flamegraph.res
Files opam-2.3.0-alpha1/libsvm.0.8.3/examples/svm_cli.ml and opam+ocaml-patch/libsvm.0.8.3/examples/svm_cli.ml differ
Files opam-2.3.0-alpha1/libsvm.0.8.3/lib/libsvm.ml and opam+ocaml-patch/libsvm.0.8.3/lib/libsvm.ml differ
Only in opam-2.3.0-alpha1/mikmatch.1.0.7: Makefile.orig
Only in opam-2.3.0-alpha1/mikmatch.1.0.7: Makefile.rej
Files opam-2.3.0-alpha1/nocrypto.0.5.4-1/_tags and opam+ocaml-patch/nocrypto.0.5.4-1/_tags differ
Files opam-2.3.0-alpha1/nocrypto.0.5.4-1/pkg/META.in and opam+ocaml-patch/nocrypto.0.5.4-1/pkg/META.in differ
Files opam-2.3.0-alpha1/nocrypto.0.5.4-2/_tags and opam+ocaml-patch/nocrypto.0.5.4-2/_tags differ
Files opam-2.3.0-alpha1/nocrypto.0.5.4-2/pkg/META.in and opam+ocaml-patch/nocrypto.0.5.4-2/pkg/META.in differ
Files opam-2.3.0-alpha1/ocp-indent.0.6.0/src/ocp-indent.ocp and opam+ocaml-patch/ocp-indent.0.6.0/src/ocp-indent.ocp differ
Files opam-2.3.0-alpha1/ocp-indent.0.6.2/src/ocp-indent.ocp and opam+ocaml-patch/ocp-indent.0.6.2/src/ocp-indent.ocp differ
Files opam-2.3.0-alpha1/ocp-indent.0.9.0/src/ocp-indent.ocp and opam+ocaml-patch/ocp-indent.0.9.0/src/ocp-indent.ocp differ
Files opam-2.3.0-alpha1/ocp-index.1.1.4/opam and opam+ocaml-patch/ocp-index.1.1.4/opam differ
Files opam-2.3.0-alpha1/ocp-index.1.1.4/src/grepMain.ml and opam+ocaml-patch/ocp-index.1.1.4/src/grepMain.ml differ
Files opam-2.3.0-alpha1/ocp-index.1.1.4/src/indexBuild.ml and opam+ocaml-patch/ocp-index.1.1.4/src/indexBuild.ml differ
Files opam-2.3.0-alpha1/ocp-index.1.1.4/src/indexOut.ml and opam+ocaml-patch/ocp-index.1.1.4/src/indexOut.ml differ
Files opam-2.3.0-alpha1/ocp-index.1.1.4/src/indexPredefined.ml and opam+ocaml-patch/ocp-index.1.1.4/src/indexPredefined.ml differ

I hope to finish it next week. This will require #19, allowing spaces instead of tabs between the date and the filename in some cases (that's going to be annoying but i have ideas on how to fix it) and allowing contextual diffs.
This PR fixes #18 already and it seems to work quite well. I'll improve the API to be able to show when a non-exact diff was applied later.

If anyone wants to help this work: The best way to help would be to add patches and expected results to the testsuite. I've done my best to give an example of package failing before succeeding after for most commits.
I'll also drop my local testsuite (opam-repository) next week but that one will have to live as a script to be run manually.

[kit-ty-kate]

I'm done i think, there are no differences between the results of the patches applications from opam-repository with GNU patch and with ocaml-patch, except when there are context diffs involved which will now fail.
I give up trying to support context diff and raise an error instead, the rest of the opam maintainers agreed to stop supporting the context diff format which isn't used anywhere else other than very old packages.

Next i'll do a cleanup, rebase and this should be good to merge and released as an alpha version for further testing during the dev and alpha stages of opam 2.4.0. Once those stages successfully passed i think we should be able to release patch.3.0.0 around mid to late April.

[hannesm]

I reviewed commit-by-commit, and am happy about your PR. Please feel free to merge & release.

[kit-ty-kate]

Thanks for the review!