fetch-pack: use unsigned int for hash_algo variable

fetch-pack: use unsigned int for hash_algo variable

hash_algo_by_name() returns unsigned int, but it is stored in
hash_algo variable as int. This goes unnoticed because of:

	DISABLE_SIGN_COMPARE_WARNINGS

On 'fetch-pack.c'

On a subsequent commit this function will be moved to 'connect.c' that
would notice this.

Change hash_algo variable type to match its return type, also make it
const because they are never modified.

Signed-off-by: Pablo Sabater <redacted>
Signed-off-by: Junio C Hamano <redacted>