update instructions to build using only MSYS2 and debug using vscode

[FuPeiJiang]

this should maybe be a PR instead

prerequisites: pacman -S --needed base-devel mingw-w64-x86_64-gcc mingw-w64-x86_64-gdb mingw-w64-x86_64-curl mingw-w64-x86_64-pcre2

build:
cmd.exe

set MSYSTEM=MINGW64
make

git-sdk-64/ is 6.35 GB
msys64/ is 1.21 GB

---

set MSYSTEM=MINGW64 and packages mingw-w64-x86_64-curl mingw-w64-x86_64-pcre2 were what were missing for me

---

debugging:

I wasn't able to build using visual studio, but I was able to debug using vscode (gdb) so I'm fine
maybe add this to wiki/README too, next to visual studio
: you can debug using vscode (gdb), here is a sample launch.json config

{
  "name": "(gdb) C git",
  "type": "cppdbg",
  "request": "launch",
  "program": "${workspaceFolder}\\git.exe",
  "args": [
    "rev-parse",
    "HEAD",
  ],
  "stopAtEntry": true,
  "cwd": "${workspaceFolder}",
  "environment": [],
  "externalConsole": false,
  "MIMode": "gdb",
  "setupCommands": [
    {
      "description": "Enable pretty-printing for gdb",
      "text": "-enable-pretty-printing",
      "ignoreFailures": true
    }
  ],
}

[dscho]

I wonder how this relates to (or competes with) https://github.com/git-for-windows/git/wiki/Building-Git?