packfile: use buffer pool for diffs

[strib]

This reuses memory slightly more efficiently, and bufPool already exists so might as well use it here.

[codecov]

Codecov Report

Merging #631 into master will decrease coverage by 0.55%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #631      +/-   ##
==========================================
- Coverage   78.28%   77.72%   -0.56%     
==========================================
  Files         130      130              
  Lines       10221    10229       +8     
==========================================
- Hits         8001     7950      -51     
- Misses       1358     1430      +72     
+ Partials      862      849      -13

Impacted Files	Coverage Δ
plumbing/format/packfile/diff_delta.go	91.15% <100%> (+0.67%)	⬆️
plumbing/transport/ssh/common.go	20.54% <0%> (-45.21%)	⬇️
plumbing/transport/ssh/auth_method.go	31.57% <0%> (-22.81%)	⬇️

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