ci: upgrade git on appveyor to version v2.21.0.windows.1

.appveyor.yml

@@ -95,6 +95,16 @@ init:
  # Load utility functions
  - ps: Invoke-Expression $env:PS_UTILS
 
+ # Upgrade git.
+ # TODO: remove when Appveyor upgrades to version 2.19.2.windows.1 or higher.
+ - ps: |-
+ $git_setup_uri = "https://github.com/git-for-windows/git/releases/" +
+ "download/v2.21.0.windows.1/Git-2.21.0-64-bit.exe"
+ Invoke-WebRequest -Uri $git_setup_uri -OutFile "$env:TEMP\git-setup.exe"
+ Start-Process -FilePath "$env:TEMP\git-setup.exe" `
+ -ArgumentList "/verysilent" `
+ -Wait
+
  # Make git check out symlinks (not placeholder text files).
  - git config --global core.symlinks true