Flatten temporary data structures in jest-diff

[pedrottimark]

Summary

Improve in small ways now and mainly reduce complexity of code diff in future pull request.

• For Diff replace {diff: string, isDifferent: boolean} with string | null

• In formatChunks and formatHunks return if no differences before initializing variables

• In formatChunks replace map with reduce to eliminate some string concatenation and mainly be able in the future to display differences within adjacent pairs of removed and added chunks

Test plan

Existing tests verify no change to behavior of package.

[codecov-io]

Codecov Report

Merging #4469 into master will decrease coverage by 0.02%.
The diff coverage is 94.73%.

@@            Coverage Diff             @@
##           master    #4469      +/-   ##
==========================================
- Coverage   56.86%   56.84%   -0.03%     
==========================================
  Files         186      186              
  Lines        6299     6298       -1     
  Branches        3        3              
==========================================
- Hits         3582     3580       -2     
- Misses       2716     2717       +1     
  Partials        1        1

Impacted Files	Coverage Δ
packages/jest-diff/src/diff_strings.js	98.88% <94.73%> (-1.12%)	⬇️

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[github-actions]

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