Skip cloning PR repository in case of no projects were changed

[cucxabong]

Implement #967 with github/gitlab support at this time. Thank @cket because of #973 , it give me an idea on how atlantis works

[codecov]

Codecov Report

Merging #1040 into master will decrease coverage by 0.79%.
The diff coverage is 30.10%.

@@            Coverage Diff             @@
##           master    #1040      +/-   ##
==========================================
- Coverage   71.49%   70.70%   -0.80%     
==========================================
  Files          67       67              
  Lines        5564     5632      +68     
==========================================
+ Hits         3978     3982       +4     
- Misses       1272     1333      +61     
- Partials      314      317       +3     

Impacted Files	Coverage Δ
cmd/server.go	79.72% <ø> (ø)
server/events/vcs/azuredevops_client.go	65.55% <0.00%> (-1.28%)	⬇️
server/events/vcs/bitbucketcloud/client.go	46.04% <0.00%> (-1.37%)	⬇️
server/events/vcs/bitbucketserver/client.go	34.83% <0.00%> (-0.81%)	⬇️
server/events/vcs/github_client.go	75.39% <0.00%> (-5.51%)	⬇️
server/events/vcs/gitlab_client.go	36.64% <0.00%> (-3.03%)	⬇️
server/events/vcs/not_configured_vcs_client.go	0.00% <0.00%> (ø)
server/events/vcs/proxy.go	0.00% <0.00%> (ø)
server/user_config.go	100.00% <ø> (ø)
server/events/project_command_builder.go	77.43% <18.18%> (-5.15%)	⬇️
... and 3 more

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[cucxabong]

@lkysow could you have sometime on this

[lkysow]

Closed by #1158. Thanks! I just made some small changes to your work.