upload-pack: look up "want" lines via commit-graph

During packfile negotiation the client will send "want" and "want-ref" lines to the server to tell it which objects it is interested in. The server-side parses each of those and looks them up to see whether it actually has requested objects. This lookup is performed by calling `parse_object()` directly, which thus hits the object database. In the general case though most of the objects the client requests will be commits. We can thus try to look up the object via the commit-graph opportunistically, which is much faster than doing the same via the object database. Refactor parsing of both "want" and "want-ref" lines to do so. The following benchmark is executed in a repository with a huge number of references. It uses cached request from git-fetch(1) as input to git-upload-pack(1) that contains about 876,000 "want" lines: Benchmark 1: HEAD~ Time (mean ± σ): 7.113 s ± 0.028 s [User: 6.900 s, System: 0.662 s] Range (min … max): 7.072 s … 7.168 s 10 runs Benchmark 2: HEAD Time (mean ± σ): 6.622 s ± 0.061 s [User: 6.452 s, System: 0.650 s] Range (min … max): 6.535 s … 6.727 s 10 runs Summary 'HEAD' ran 1.07 ± 0.01 times faster than 'HEAD~' Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>