Add `-H`/`--sync-github-prs` and `-L`/`--sync-gitlab-mrs` flags to `traverse`

[PawelLipski]

• [ ] Update descriptions of PRs/MRs even if PR base is the same as in .git/machete => separate option, --update-all-descriptions => open an issue
• [x] Restack instead of plain retarget => future release
• [x] Warn if multiple PRs have the current branch as head

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.51%. Comparing base (cf7be3c) to head (55aacd2).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1340      +/-   ##
===========================================
+ Coverage    98.47%   98.51%   +0.04%     
===========================================
  Files           14       14              
  Lines         3992     4038      +46     
  Branches       780      789       +9     
===========================================
+ Hits          3931     3978      +47     
+ Misses          39       38       -1     
  Partials        22       22              

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