shlib.sh: set var_have_git_2101 for Git v2.10.1 or later
Prior to Git's
b773ddea2cd3b08c ("pack-objects: walk tag chains for
--include-tag", 2016-09-07, v2.10.1), using the --include-tag
option with pack-objects could result in a corrupted pack.
(It might contain tags that reference unknown tag objects.)
Therefore even though the --include-tag option has been available
since Git v1.5.5, provide a convenient variable that can be tested
to avoid using the option when it could still produce corrupted packs.
Signed-off-by: Kyle J. McKay <mackyle@gmail.com>