sha1_name: get_sha1_with_context learns to follow symlinks

sha1_name: get_sha1_with_context learns to follow symlinks

Wire up get_sha1_with_context to call get_tree_entry_follow_symlinks
when GET_SHA1_FOLLOW_SYMLINKS is passed in flags. G_S_FOLLOW_SYMLINKS
is incompatible with G_S_ONLY_TO_DIE because the diagnosis
that ONLY_TO_DIE triggers does not at present consider symlinks, and
it would be a significant amount of additional code to allow it to
do so.

Signed-off-by: David Turner <dturner@twopensource.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>