Per git-config's man page, include.* directives are not respected by
default when asked to evaluate a specific file (whether via --global,
--file /path/to/.gitconfig, or GIT_CONFIG=/path/to/.gitconfig).
This patch ensures that, if GIT_CONFIG is set in the environment when
e.g. git-lfs-push is invoked, config directives from included files are
evaluated.
This is important if, for example, an included file contains a
url.<foo>.insteadOf directive or lfs.<foo>.locksverify directive that's
shared between many repositories.
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