Allow specifying command to apply patches

[MisterDA]

The patches field allows listing patches that are applied sequentially to the source as with the patch command, however not all patches are created equal. For instance, Git has a custom format for binary diffs that the venerable patch utility doesn't understand. In order to support such patches, the patch utility should be changed to git apply (although I haven't yet checked if that works outside a git repo). An example of a binary file that cannot be patched easily today is the configure script from the OCaml compiler.

[rjbou]

It is not possible ftm to specify a patch command, as for make, curl, etc. [g]patch is an opam required tool, but even if git is widely used, it is not required by opam. There is several issues where pops the idea to use an ocaml written patch, to get ride of some patch limits (see #3782 (comment)).

[dra27]

Early versions of the Windows patches used git apply during 2.0 development... it was unreliable which is why the patch rewriter got added. It just swaps one set of problems for another 😉

@rjbou has referenced the tracking issue - for the time being, it's better to amend the patch - see, for example, https://github.com/ocaml/opam/blob/master/shell/re-patch.sh

[dra27]

Closing this in favour of the main patch issue