Add arm64 support to git-wrapper

[dennisameling]

CC @dscho

Builds further on #43

Some of the logic in this PR comes from https://www.rudyhuyn.com/blog/2017/12/13/how-to-detect-that-your-x86-application-runs-on-windows-on-arm/

This PR allows users to build mingw32 but with a special git-wrapper that supports arm64 on Windows. This way, the regular mingw32 packages can be used, while Git itself (e.g. git clone) runs natively on arm64.

MSYSTEM is set correctly to ARM64, and after manually adding /arm64/bin to the PATH (should be done automatically when git-for-windows/git-sdk-32#6 is merged), the native Git executable is used as well:

Here you can see that the wrapper git.exe is still 32-bit, but the actual git commands are handled by the native ARM64 binary:

The folder structure is as follows in that case:

Build steps for mingw-w64-i686-git

1. Checkout this PR in C:\git-sdk-64\usr\src\MINGW-packages
2. Open Git SDK bash
3. cd /usr/src/mingw-packages/mingw-w64-git
4. MINGW_INSTALLS=mingw32 makepkg-mingw -sLf
5. Open pkg/mingw-w64-i686-git, you'll find the generated binaries here

Build steps for native Git arm64 binaries

Follow the steps mentioned here: git-for-windows/git#2915 (comment)

Putting everything together on the arm64 device

1. Download Git for Windows portable 32-bit
2. Create an arm64 folder and copy the native Git arm64 binaries from the previous step into it:

[dennisameling]

@dscho it works with IsWow64Process2! 🚀

It actually works on Windows 8.1 as well, so I assume that the logic I got from here is enough to dynamically load IsWow64Process2.

I just had to manually copy the DLLs for the generated native ARM64 binaries. Created a separate issue for that: git-for-windows/git#2970

I double-checked and both the git.exe triggered from Git Bash and from the Windows cmd line (see below) are the native git.exe binaries 👍🏼

Windows 10 ARM64:

Windows 8.1 VM x64:

[dscho]

Thank you so much for working on this!

I suggested a couple changes, nothing big, should be pretty close to being done now. Oh, and one more suggestion: could you squash your commits into a single one?

[dennisameling]

@dscho just implemented the feedback and squashed everything into a single commit 🚀

Can confirm things still work on my ARM64 device as well as in my Windows 8.1 VM (which of course doesn't have IsWow64Process2, so at least things don't crash or anything).

UPDATE here's a screenshot from ARM64, all looks good as far as I can see:

[dscho]

From your comments, I gather that you tested this, and since everything looks pretty and neat, I will merge it. Thank you so much!

[dscho]

@dennisameling I got a report that the 32-bit version of Git for Windows v2.30.1 is completely broken on ARM64 (fork bomb). Could you verify that? I suspect that this PR is responsible for that issue.

[dscho]

@dennisameling I got a report that the 32-bit version of Git for Windows v2.30.1 is completely broken on ARM64 (fork bomb). Could you verify that? I suspect that this PR is responsible for that issue.

Here is my attempt to help: #46. But I cannot test (I don't have access to any Windows/ARM64 system).