builtin/blame: fix type of `length` variable when emitting object ID

builtin/blame: fix type of `length` variable when emitting object ID

The `length` variable is used to store how many bytes we wish to emit
from an object ID. This value will either be the full hash algorithm's
length, or the abbreviated hash that can be set via `--abbrev` or the
"core.abbrev" option. The former is of type `size_t`, whereas the latter
is of type `int`, which causes a warning with "-Wsign-compare".

The reason why `abbrev` is using a signed type is mostly that it is
initialized with `-1` to indicate that we have to compute the minimum
abbreviation length. This length is computed via `find_alignment()`,
which always gets called before `emit_other()`, and thus we can assume
that the value would never be negative in `emit_other()`.

In fact, we can even assume that the value will always be at least
`MINIMUM_ABBREV`, which is enforced by both `git_default_core_config()`
and `parse_opt_abbrev_cb()`. We implicitly rely on this by subtracting
up to 3 without checking for whether the value becomes negative. We then
pass the value to printf(3p) to print the prefix of our object's ID, so
if that assumption was violated we may end up with undefined behaviour.

Squelch the warning by asserting this invariant and casting the value of
`abbrev` to `size_t`. This allows us to store the whole length as an
unsigned integer, which we can then pass to `fwrite()`.

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