sha1_name: parse less while finding common prefix

sha1_name: parse less while finding common prefix

Create get_hex_char_from_oid() to parse oids one hex character at a
time. This prevents unnecessary copying of hex characters in
extend_abbrev_len() when finding the length of a common prefix.

Signed-off-by: Derrick Stolee <redacted>
Reviewed-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>