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.
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