pretty: lazy-load commit data when expanding user-format

When we expand a user-format, we try to avoid work that isn't necessary
for the output. For instance, we don't bother parsing the commit header
until we know we need the author, subject, etc.

But we do always load the commit object's contents from disk, even if
the format doesn't require it (e.g., just "%H"). Traditionally this
didn't matter much, because we'd have loaded it as part of the traversal
anyway, and we'd typically have those bytes attached to the commit
struct (or these days, cached in a commit-slab).

But when we have a commit-graph, we might easily get to the point of
pretty-printing a commit without ever having looked at the actual object
contents. We should push off that load (and reencoding) until we're
certain that it's needed.

I think the results of p4205 show the advantage pretty clearly (we serve
parent and tree oids out of the commit struct itself, so they benefit as
well):

# using git.git as the test repo
Test HEAD^ HEAD
----------------------------------------------------------------------
4205.1: log with %H 0.40(0.39+0.01) 0.03(0.02+0.01) -92.5%
4205.2: log with %h 0.45(0.44+0.01) 0.09(0.09+0.00) -80.0%
4205.3: log with %T 0.40(0.39+0.00) 0.04(0.04+0.00) -90.0%
4205.4: log with %t 0.46(0.46+0.00) 0.09(0.08+0.01) -80.4%
4205.5: log with %P 0.39(0.39+0.00) 0.03(0.03+0.00) -92.3%
4205.6: log with %p 0.46(0.46+0.00) 0.10(0.09+0.00) -78.3%
4205.7: log with %h-%h-%h 0.52(0.51+0.01) 0.15(0.14+0.00) -71.2%
4205.8: log with %an-%ae-%s 0.42(0.41+0.00) 0.42(0.41+0.01) +0.0%

# using linux.git as the test repo
Test HEAD^ HEAD
----------------------------------------------------------------------
4205.1: log with %H 7.12(6.97+0.14) 0.76(0.65+0.11) -89.3%
4205.2: log with %h 7.35(7.19+0.16) 1.30(1.19+0.11) -82.3%
4205.3: log with %T 7.58(7.42+0.15) 1.02(0.94+0.08) -86.5%
4205.4: log with %t 8.05(7.89+0.15) 1.55(1.41+0.13) -80.7%
4205.5: log with %P 7.12(7.01+0.10) 0.76(0.69+0.07) -89.3%
4205.6: log with %p 7.38(7.27+0.10) 1.32(1.20+0.12) -82.1%
4205.7: log with %h-%h-%h 7.81(7.67+0.13) 1.79(1.67+0.12) -77.1%
4205.8: log with %an-%ae-%s 7.90(7.74+0.15) 7.81(7.66+0.15) -1.1%

I added the final test to show where we don't improve (the 1% there is
just lucky noise), but also as a regression test to make sure we're not
doing anything stupid like loading the commit multiple times when there
are several placeholders that need it.

Reported-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>