Add ability to skip cloning if no Terraform is modified

[cket]

Related to #967. Adds skip-clone-no-tf flag to control this behavior. I'm not sure if this will exactly satisfy the requirements in the ticket since it does not inspect atlantis.yaml but should be a step in the right direction

A couple benefits to this addition:

• not specific to any VCS platform
• improved performance for non-TF PRs - would help Atlantis scale with larger amounts of non-TF PRs
• prevents certain Atlantis errors on non-TF PRs by avoiding the need for the lock in those situations (workspace is currently locked by another command that is running for this pull request)

Let me know if I'm missing anything or if this is helpful

[codecov]

Codecov Report

Merging #973 into master will increase coverage by 0.04%.
The diff coverage is 84.61%.

@@            Coverage Diff             @@
##           master     #973      +/-   ##
==========================================
+ Coverage   72.12%   72.16%   +0.04%     
==========================================
  Files          65       65              
  Lines        5316     5325       +9     
==========================================
+ Hits         3834     3843       +9     
  Misses       1186     1186              
  Partials      296      296

Impacted Files	Coverage Δ
server/user_config.go	100% <ø> (ø)	⬆️
cmd/server.go	79.26% <ø> (ø)	⬆️
server/server.go	63.66% <100%> (+0.1%)	⬆️
server/events/project_finder.go	94.56% <100%> (+0.18%)	⬆️
server/events/project_command_builder.go	82.5% <77.77%> (+0.37%)	⬆️

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[lkysow]

Hey Christopher, thanks for the PR but I'm not sure this is the best approach that will solve the whole problem for all atlantis users.

Many Atlantis users use the atlantis.yaml file to determine which file patterns will trigger a plan. By only looking at .tf files these users aren't supported.

I looked into doing a clone into a temp directory and then performing the detection there before moving into the working directory but we can't simply move the cloned directory and clobber the existing working directory because we actually expect it to persist through atlantis commands run on the same pull request so that the .tfplan files aren't deleted.

Thus we'd need to clone it first into the temp dir, run our checks and then re-clone it into the proper working dir. This seems like a smell that this also isn't the right approach.

I think we either need to support queuing or go the route that @timoguin talked about in the ticket where we make an api call to get the contents of atlantis.yaml.

[cket]

Got it. I think a call to view atlantis.yaml would be preferable so we don't clone if we don't have to. It sounds like that is possible for github at least but I'm not sure if other vcs have that capability

[lkysow]

It sounds like that is possible for github at least but I'm not sure if other vcs have that capability

For those VCS's we can just keep the existing logic and always clone.

[lkysow]

Closing in favour of #1040 which supports downloading the atlantis.yaml file.