clone_mirror: refuse to clone from self

[dveeden]

What problem does this PR solve?

When running something like this:

tiup mirror clone /tmp/tidb_mirror -a amd64 --prefix v5.1.0 --os linux
tiup mirror set /tmp/tidb_mirror
tiup mirror clone /tmp/tidb_mirror -a amd64 --prefix v5.1.0 --os linux

This results in metadata being changed/corrupted.

Then the next tiup mirror clone ... results in:

Error: read manifest from mirror(/tmp/tidb_mirror) failed: not enough signatures (0) for threshold 1 in timestamp.json

This can easily be fixed with tiup mirror set ..., but it is best to
avoid this. And also cloning from self is probably not what the user
wanted in the first place. So adding the source to the output to make
this easier to spot.

Related: https://tidbcommunity.slack.com/archives/C019S7HLP61/p1625059913130200

Check List

Tests

• Manual test (add detailed scripts or steps below)

Release notes:

A clear error is given when trying to clone a mirror from itself

[codecov-commenter]

Codecov Report

Merging #1464 (d029558) into master (6d88e56) will decrease coverage by 9.58%.
The diff coverage is 0.00%.

❗ Current head d029558 differs from pull request most recent head 5ecfe38. Consider uploading reports for the commit 5ecfe38 to get more accurate results

@@            Coverage Diff             @@
##           master    #1464      +/-   ##
==========================================
- Coverage   24.85%   15.26%   -9.59%     
==========================================
  Files         267      150     -117     
  Lines       21265    13075    -8190     
==========================================
- Hits         5285     1996    -3289     
+ Misses      15173    10570    -4603     
+ Partials      807      509     -298     

Flag	Coverage Δ
integrate	15.26% <0.00%> (+0.02%)	⬆️
tiup	15.26% <0.00%> (+0.02%)	⬆️
unittest	?

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

Impacted Files	Coverage Δ
pkg/repository/clone_mirror.go	0.00% <0.00%> (ø)
embed/embed.go	0.00% <0.00%> (-100.00%)	⬇️
pkg/utils/regexp.go	0.00% <0.00%> (-100.00%)	⬇️
pkg/cluster/spec/validate.go	0.00% <0.00%> (-89.97%)	⬇️
pkg/cluster/executor/checkpoint.go	0.00% <0.00%> (-82.61%)	⬇️
pkg/utils/semver.go	2.60% <0.00%> (-82.61%)	⬇️
pkg/utils/diff.go	0.00% <0.00%> (-82.50%)	⬇️
pkg/meta/err.go	0.00% <0.00%> (-81.25%)	⬇️
pkg/cluster/api/error.go	0.00% <0.00%> (-80.00%)	⬇️
pkg/cluster/spec/parse_topology.go	0.00% <0.00%> (-78.47%)	⬇️
... and 162 more

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[AstroProfundis]

/merge

[ti-chi-bot]

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

Commit hash: 5ecfe38