var: add GIT_DEFAULT_BRANCH variable

var: add GIT_DEFAULT_BRANCH variable

Introduce the logical variable GIT_DEFAULT_BRANCH which represents the
the default branch name that will be used by "git init".

Currently this variable is equivalent to
    git config init.defaultbranch || 'master'

This however will break if at one point the default branch is changed as
indicated by `default_branch_name_advice` in `refs.c`.

By providing this command ahead of time users of git can make their
code forward-compatible.

Helped-by: Johannes Schindelin <redacted>
Helped-by: Eric Sunshine <redacted>
Helped-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Thomas Weißschuh <redacted>
Signed-off-by: Junio C Hamano <redacted>