Use io.CopyBuffer instead of io.ReadAll

[k1LoW]

Only if there is a cache hit, but memory efficiency is improved.

[github-actions]

Code Metrics Report

	main (1439b7d)	#79 (783ecd1)	+/-
Coverage	61.2%	61.9%	+0.7%
Code to Test Ratio	1:2.2	1:2.2	-0.0
Test Execution Time	40s	39s	-1s

Details

  |                     | main (1439b7d) | #79 (783ecd1) |  +/-  |
  |---------------------|----------------|---------------|-------|
+ | Coverage            |          61.2% |         61.9% | +0.7% |
  |   Files             |              7 |             8 |    +1 |
  |   Lines             |            446 |           454 |    +8 |
+ |   Covered           |            273 |           281 |    +8 |
- | Code to Test Ratio  |          1:2.2 |         1:2.2 |  -0.0 |
  |   Code              |            758 |           773 |   +15 |
  |   Test              |           1685 |          1685 |     0 |
+ | Test Execution Time |            40s |           39s |   -1s |

Code coverage of files in pull request scope (76.2% → 77.9%)

Files	Coverage	+/-
copybuf.go	80.0%	+80.0%
rc.go	77.8%	+1.6%

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Reported by octocov