makepkg: Add GITFLAGS environmental variable to customise checkout
The default flag used to clone a git repository when using makepkg is "--mirror". However, when working with huge repositories, the use of different flags during cloning can allow an faster checkout. For example, using "--filter=blob:none" allows for small checkouts, at the expense of requiring downloads during the build stage if anything but the HEAD commit is used for the build. In addition, this example would serve as a replacement for the often requested (but broken) addition of --depth=1. Add support for the environment variable GITFLAG to pass flags for the git clone command. Note that this overrides the default rather than adding to it in order to prevent incompatibilities.