Disable most caching for packfile objects

[jtibshirani]

By default go-git maintains an LRU cache of git objects of size 96MB. When an object's contents are loaded, it's stored as a MemoryObject in this cache. This cache is not super useful in the indexing access pattern, which accesses each file only once. And in many profiles, we see a substantial number of allocations from these memory objects.

This PR disables caching for most git objects by setting LargeObjectThreshold: 1. go-git still proactively caches packfile objects under 16KB (see smallObjectThreshold here).

Follow up to #852. This change is also gated by the ZOEKT_ENABLE_GOGIT_OPTIMIZATION feature flag.

[jtibshirani]

Here's the difference in total allocations for indexing megarepo. It's a small but consistent improvement. I also like the general strategy of being more "vanilla" with our git use (eventually moving away from go-git). This PR helps take us in that direction.

~/code/zoekt % go tool pprof -alloc_space -diff_base base.prof large1.prof
File: zoekt-git-index
Type: alloc_space
Time: Oct 29, 2024 at 12:16pm (PDT)
Entering interactive mode (type "help" for commands, "o" for options)
(pprof) top10
Showing nodes accounting for -1.72GB, 3.51% of 49.12GB total
Dropped 246 nodes (cum <= 0.25GB)
Showing top 10 nodes out of 122
      flat  flat%   sum%        cum   cum%
   -3.17GB  6.46%  6.46%    -3.17GB  6.46%  github.com/go-git/go-git/v5/plumbing.(*MemoryObject).Write
   -2.63GB  5.35% 11.81%    -2.63GB  5.35%  github.com/sourcegraph/zoekt.(*postingsBuilder).newSearchableString
    1.53GB  3.12%  8.69%    -1.63GB  3.32%  io.copyBuffer
    1.01GB  2.07%  6.63%     1.01GB  2.07%  compress/flate.(*dictDecoder).init (inline)
    0.51GB  1.05%  5.58%     0.51GB  1.05%  github.com/go-git/go-git/v5/utils/sync.init.func2
    0.38GB  0.77%  4.81%     0.38GB  0.77%  bytes.NewReader
    0.35GB   0.7%  4.10%     0.35GB   0.7%  bufio.NewReaderSize
    0.30GB  0.61%  3.49%     0.30GB  0.61%  io.LimitReader
   -0.24GB  0.49%  3.98%    -0.36GB  0.74%  github.com/sourcegraph/go-ctags.(*ctagsProcess).Parse
    0.23GB  0.47%  3.51%     1.24GB  2.53%  compress/flate.NewReader

This change was motivated by several "Allocated Heap" profiles we see on GCP, where MemoryObject.Write dominates allocations:

[keegancsmith]

yay!