add repo path functions

[tbell83]

This PR adds two functions, get_path_to_repo_root() and get_path_from_repo_root(), loosely linked to #305

Tests:

terragrunt/test on  add-repo-path-functions via 🐹 v1.17
❯ go test -v -run TestGetPathToRepoRoot
=== RUN   TestGetPathToRepoRoot
=== PAUSE TestGetPathToRepoRoot
=== CONT  TestGetPathToRepoRoot
    integration_test.go:3467: Copying fixture-get-path-to-repo-root to /var/folders/mg/dlfyyxbj4s32mwd8599bh3200000gq/T/terragrunt-test1054949920
runTerragruntVersionCommand after split
[terragrunt apply-all --terragrunt-non-interactive --terragrunt-working-dir /private/var/folders/mg/dlfyyxbj4s32mwd8599bh3200000gq/T/terragrunt-test1054949920/fixture-get-path-to-repo-root]
WARN[0000] 'apply-all' is deprecated. Running 'terragrunt run-all apply' instead. Please update your workflows to use 'terragrunt run-all apply', as 'apply-all' may be removed in the future!
INFO[0000] The stack at /private/var/folders/mg/dlfyyxbj4s32mwd8599bh3200000gq/T/terragrunt-test1054949920/fixture-get-path-to-repo-root will be processed in the following order for command apply:
Group 1
- Module /private/var/folders/mg/dlfyyxbj4s32mwd8599bh3200000gq/T/terragrunt-test1054949920/fixture-get-path-to-repo-root
  prefix=[/private/var/folders/mg/dlfyyxbj4s32mwd8599bh3200000gq/T/terragrunt-test1054949920/fixture-get-path-to-repo-root]

Initializing the backend...

Initializing provider plugins...

Terraform has been successfully initialized!

You may now begin working with Terraform. Try running "terraform plan" to see
any changes that are required for your infrastructure. All Terraform commands
should now work.

If you ever set or change modules or backend configuration for Terraform,
rerun this command to reinitialize your working directory. If you forget, other
commands will detect it and remind you to do so if necessary.

Changes to Outputs:
  + path_to_root = "../../"

You can apply this plan to save these new output values to the Terraform
state, without changing any real infrastructure.

Apply complete! Resources: 0 added, 0 changed, 0 destroyed.

Outputs:

path_to_root = "../../"
runTerragruntVersionCommand after split
[terragrunt output -no-color -json --terragrunt-non-interactive --terragrunt-working-dir /private/var/folders/mg/dlfyyxbj4s32mwd8599bh3200000gq/T/terragrunt-test1054949920/fixture-get-path-to-repo-root]
--- PASS: TestGetPathToRepoRoot (15.09s)
PASS
ok  	github.com/gruntwork-io/terragrunt/test	16.128s
terragrunt/test on  add-repo-path-functions via 🐹 v1.17 took 22s
❯ go test -v -run TestGetPathFromRepoRoot
=== RUN   TestGetPathFromRepoRoot
=== PAUSE TestGetPathFromRepoRoot
=== CONT  TestGetPathFromRepoRoot
    integration_test.go:3467: Copying fixture-get-path-from-repo-root to /var/folders/mg/dlfyyxbj4s32mwd8599bh3200000gq/T/terragrunt-test2705825776
runTerragruntVersionCommand after split
[terragrunt apply-all --terragrunt-non-interactive --terragrunt-working-dir /private/var/folders/mg/dlfyyxbj4s32mwd8599bh3200000gq/T/terragrunt-test2705825776/fixture-get-path-from-repo-root]
WARN[0000] 'apply-all' is deprecated. Running 'terragrunt run-all apply' instead. Please update your workflows to use 'terragrunt run-all apply', as 'apply-all' may be removed in the future!
INFO[0000] The stack at /private/var/folders/mg/dlfyyxbj4s32mwd8599bh3200000gq/T/terragrunt-test2705825776/fixture-get-path-from-repo-root will be processed in the following order for command apply:
Group 1
- Module /private/var/folders/mg/dlfyyxbj4s32mwd8599bh3200000gq/T/terragrunt-test2705825776/fixture-get-path-from-repo-root
  prefix=[/private/var/folders/mg/dlfyyxbj4s32mwd8599bh3200000gq/T/terragrunt-test2705825776/fixture-get-path-from-repo-root]

Initializing the backend...

Initializing provider plugins...

Terraform has been successfully initialized!

You may now begin working with Terraform. Try running "terraform plan" to see
any changes that are required for your infrastructure. All Terraform commands
should now work.

If you ever set or change modules or backend configuration for Terraform,
rerun this command to reinitialize your working directory. If you forget, other
commands will detect it and remind you to do so if necessary.

Changes to Outputs:
  + path_from_root = "terragrunt-test2705825776/fixture-get-path-from-repo-root"

You can apply this plan to save these new output values to the Terraform
state, without changing any real infrastructure.

Apply complete! Resources: 0 added, 0 changed, 0 destroyed.

Outputs:

path_from_root = "terragrunt-test2705825776/fixture-get-path-from-repo-root"
runTerragruntVersionCommand after split
[terragrunt output -no-color -json --terragrunt-non-interactive --terragrunt-working-dir /private/var/folders/mg/dlfyyxbj4s32mwd8599bh3200000gq/T/terragrunt-test2705825776/fixture-get-path-from-repo-root]
--- PASS: TestGetPathFromRepoRoot (12.21s)
PASS
ok  	github.com/gruntwork-io/terragrunt/test	12.826s

[yorinasub17]

Makes sense and LGTM! Thanks for your contribution! Just have one minor suggestion.

[tbell83]

Updated to address comments

[yorinasub17]

Thanks for addressing the comments and apologies for the delay in getting to this. I've been buried with other tasks and only now getting back to terragrunt again.

The updates LGTM! I'll kick off a regression build and if it passes, we can merge this in.

[yorinasub17]

Build passed, so will go ahead and merge this in! Thanks again for your contribution!

[rhoboat]

+1 to this add! Thank you!