shlib.sh: set var_have_git_210 for Git v2.1.0 or later
When doing fancier repacking in the future, we will need to know
whether or not pack bitmaps are supported.
Technically they've been supported since v2.0.0 although they
probably should not be used before v2.1.4.
But, we activate them with repack.writeBitmaps=true which is only
understood by v2.1.0 or later and we want to match the same behavior
as that config file setting provides. Therefore we test for v2.1.0
or later instead of v2.0.0 or later.
Signed-off-by: Kyle J. McKay <mackyle@gmail.com>