http: simplify parsing of remote objects/info/packs

http: simplify parsing of remote objects/info/packs

We can use skip_prefix() and parse_oid_hex() to continuously increment
our pointer, rather than dealing with magic numbers. This also fixes a
few small shortcomings:

  - if we see a line with the right prefix, suffix, and length, i.e.
    matching /P pack-.{40}.pack\n/, we'll interpret the middle part as
    hex without checking if it could be parsed. This could lead to us
    looking at uninitialized garbage in the hash array. In practice this
    means we'll just make a garbage request to the server which will
    fail, though it's interesting that a malicious server could convince
    us to leak 40 bytes of uninitialized stack to them.

  - the current code is picky about seeing a newline at the end of file,
    but we can easily be more liberal

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>