sdk-installer: ensure that Git for Windows' keyring is imported

This is required so that the SDK can be installed fully, now that we
sign each and every package in Git for Windows' Pacman repository.

As the post-install script of msys2-keyring destroys every previous
attempt to initialize Pacman's keyring, we need to install
msys2-keyring (and gnupg) specifically before adding Git for Windows'
keys.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

sdk-installer/setup-git-sdk.bat

@@ -104,6 +104,25 @@
 
 @IF ERRORLEVEL 1 GOTO INSTALL_INFO
 
+@SET /A counter=0
+:INSTALL_MSYS2_KEYRING
+@SET /A counter+=1
+@IF %counter% GEQ 5 @(
+	@ECHO Could not install msys2-keyring
+	@PAUSE
+	@EXIT 1
+)
+
+@REM next, force update msys2-keyring
+@"%cwd%"\usr\bin\pacman -S --needed --force --noconfirm gnupg msys2-keyring
+
+@IF ERRORLEVEL 1 GOTO INSTALL_MSYS2_KEYRING
+
+@REM now, add Git for Windows' keyring
+@"%cwd%"\usr\bin\bash.exe -l -c ^
+	'/usr/bin/bash /usr/bin/pacman-key --populate git-for-windows'
+@IF ERRORLEVEL 1 PAUSE
+
 @SET /A counter=0
 :INSTALL_PACMAN
 @SET /A counter+=1