builtin/blame: don't access potentially unitialized `the_hash_algo`

builtin/blame: don't access potentially unitialized `the_hash_algo`

We access `the_hash_algo` in git-blame(1) before we have executed
`parse_options_start()`, which may not be properly set up in case we
have no repository. This is fine for most of the part because all the
call paths that lead to it (git-blame(1), git-annotate(1) as well as
git-pick-axe(1)) specify `RUN_SETUP` and thus require a repository.

There is one exception though, namely when passing `-h` to print the
help. Here we will access `the_hash_algo` even if there is no repo.
This works fine right now because `the_hash_algo` gets sets up to point
to the SHA1 algorithm via `initialize_repository()`. But we're about to
stop doing this, and thus the code would lead to a `NULL` pointer
exception.

Prepare the code for this and only access `the_hash_algo` after we are
sure that there is a proper repository.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>