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
• Restack instead of plain retarget => future release
• Warn if multiple PRs have the current branch as head

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 40.29851% with 40 lines in your changes missing coverage. Please review.

Project coverage is 97.42%. Comparing base (6fa8c71) to head (53f8c97).

Files with missing lines	Patch %	Lines
git_machete/client.py	32.20%	35 Missing and 5 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1340      +/-   ##
===========================================
- Coverage    98.42%   97.42%   -1.00%     
===========================================
  Files           14       14              
  Lines         3881     3929      +48     
  Branches       849      859      +10     
===========================================
+ Hits          3820     3828       +8     
- Misses          39       74      +35     
- Partials        22       27       +5     

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