tiup: optimize some mirror subcommands

[AstroProfundis]

What problem does this PR solve?

• Optimize some help messages and command outputs to better describe what's going on when users try to initialize a new repository
• Extend relative paths to absolute ones on mirror set

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation

Release notes:

tiup: optimize some mirror subcommands

[codecov-commenter]

Codecov Report

Merging #1331 (503b16b) into master (334c38a) will increase coverage by 30.06%.
The diff coverage is 71.42%.

@@             Coverage Diff             @@
##           master    #1331       +/-   ##
===========================================
+ Coverage   25.86%   55.93%   +30.06%     
===========================================
  Files         264      290       +26     
  Lines       19703    21580     +1877     
===========================================
+ Hits         5097    12070     +6973     
+ Misses      13825     7618     -6207     
- Partials      781     1892     +1111     

Flag	Coverage Δ
cluster	43.28% <ø> (?)
dm	25.63% <ø> (?)
integrate	50.36% <71.42%> (+34.61%)	⬆️
playground	14.47% <ø> (?)
tiup	15.79% <71.42%> (+0.04%)	⬆️
unittest	22.42% <7.14%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
cmd/mirror.go	48.93% <71.42%> (+0.32%)	⬆️
pkg/cluster/api/error.go	75.00% <0.00%> (-5.00%)	⬇️
pkg/cluster/operation/operation.go	78.26% <0.00%> (ø)
pkg/logger/logger.go	100.00% <0.00%> (ø)
pkg/cluster/operation/telemetry.go	0.00% <0.00%> (ø)
components/playground/instance/process.go	81.39% <0.00%> (ø)
components/playground/instance/pump.go	0.00% <0.00%> (ø)
components/playground/instance/tiflash.go	59.09% <0.00%> (ø)
pkg/cluster/operation/destroy.go	56.61% <0.00%> (ø)
pkg/logger/debug.go	16.66% <0.00%> (ø)
... and 211 more

---

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[lucklove]

/lgtm

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• lucklove

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[lucklove]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: c1b57a4

[AstroProfundis]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 503b16b