Fix msys2-runtime/update-patches.sh format-patch
Fix msys2-runtime/update-patches.sh format-patch

Handle the case that `^HEAD^{/Start.the.merging.rebase}` doesn't match
anything by checking before calling format-patch.

Signed-off-by: Dakota Hawkins <41574c91eec67c2c8dd95dd2aedab491489456ee@gmail.com>

git-for-windows/MSYS2-packages,dscho/MSYS2-packages,git-for-windows/MSYS2-packages,dscho/MSYS2-packages,git-for-windows/MSYS2-packages,git-for-windows/MSYS2-packages,dscho/MSYS2-packages,dscho/MSYS2-packages