Only check remote is reachable when ttl timeout.

[wudong]

What I did: only call repo.isReachable() when it is actually needed.

Why I did it: Calling repo.isReachable() each time give me a few seconds delay in my environment (WSL2/Ubuntu). Didn't check other environment.

Because the code path is executed for every invocation of the command line, it gives me a unbearable delay for using it.

[codecov-commenter]

Codecov Report

Merging #52 into master will decrease coverage by 4.33%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master      #52      +/-   ##
==========================================
- Coverage   67.03%   62.70%   -4.34%     
==========================================
  Files           3        3              
  Lines         182      185       +3     
==========================================
- Hits          122      116       -6     
- Misses         34       44      +10     
+ Partials       26       25       -1     

Impacted Files	Coverage Δ
cache/cache.go	53.33% <0.00%> (-5.76%)	⬇️

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[mstruebing]

Wow I didn't notice this behavior, thank you - great contribution 🚀