Document how to apply patch from mercurial to git

[Mariatta]

How I do it:

1. In the CPython mercurial repo, re-create the patch using --git parameter:
   hg diff --git > issueNNNN-git.patch

2. In the CPython git repo, create and switch to a new branch. Apply the patch into CPython git repository:
   git apply /path/to/issueNNNN-git.patch

3. Commit, push, and open the PR.

[Mariatta]

I think this can go in committing.rst

[DimitrisJim]

Wouldn't this be better in gitdevs.rst or gitbootcamp.rst?

[Mariatta]

gitdevs.rst (Mercurial for git developers) might go away, as proposed in #120

About adding it to gitbootcamp.rst, it's fine by me :)

[brettcannon]

Does patch -p1 < path/to/patch.diff not work?

[Mariatta]

If it works, let's use patch -p1 < path/to/patch.diff.
I'm traveling today and on my phone right now so I can't try it myself 😝

[ezio-melotti]

Several patches on the tracker have already been created with --git, so for those, the 1st step is unnecessary. git apply might also work with non---git patches, but I haven't tested.
patch is generally not available on Windows.

[terryjreedy]

I believe git patches do not work with Rietveld, so people have often been asked to resubmit. In any case, the top of the file indicates format.
Program 'patch' does not come with Windows; don't know if it exists.

[Mariatta]

Alright just got around trying out patch. It works on my Macbook.

If we want to document patch and mentioning it works for Mac & Unix only, the steps would be

1. In the CPython git repo, create and switch to a branch
2. patch -p1 < /path/to/issueNNNN.patch
3. commit, push and open the PR.

[refi64]

FWIW there's also Mozilla's hg-patch-to-git-patch, with which the entire workflow can be shortened to:

curl patch_url | hg-patch-to-git-patch | git am

[matrixise]

+1 for git apply.
in the past, I used patch with the patches from hg.python.org when I wanted to patch my working directory for a test and sometimes, I got an error.
Since I use git apply, it's really rare when I have an issue.

About hg-patch-to-git-patch maybe we could use it if we create a copy in the tools directory of CPython just for the maintenance, because if Mozilla stops the dev of this small script, we will discuss for a new tool, new way to do that.