[plugin-registry-generator] Improve plugin registry generator

[svor]

Is your task related to a problem? Please describe

For now if execute plugin-registry-generator tool in the folder which is not a git repository, it will fail with error:

stack=Error: fatal: not a git repository (or any parent up to mount point /home)
Stopping at filesystem boundary (GIT_DISCOVERY_ACROSS_FILESYSTEM not set).

    at Object.action (/home/vsvydenko/.npm/_npx/292135/lib/node_modules/@eclipse-che/plugin-registry-generator/node_modules/simple-git/src/lib/plugins/error-detection.plugin.js:30:33)
    at PluginStore.exec (/home/vsvydenko/.npm/_npx/292135/lib/node_modules/@eclipse-che/plugin-registry-generator/node_modules/simple-git/src/lib/plugins/plugin-store.js:21:33)
    at /home/vsvydenko/.npm/_npx/292135/lib/node_modules/@eclipse-che/plugin-registry-generator/node_modules/simple-git/src/lib/runners/git-executor-chain.js:95:45
    at new Promise (<anonymous>)
    at GitExecutorChain.handleTaskData (/home/vsvydenko/.npm/_npx/292135/lib/node_modules/@eclipse-che/plugin-registry-generator/node_modules/simple-git/src/lib/runners/git-executor-chain.js:93:16)
    at GitExecutorChain.<anonymous> (/home/vsvydenko/.npm/_npx/292135/lib/node_modules/@eclipse-che/plugin-registry-generator/node_modules/simple-git/src/lib/runners/git-executor-chain.js:77:46)
    at Generator.next (<anonymous>)
    at fulfilled (/home/vsvydenko/.npm/_npx/292135/lib/node_modules/@eclipse-che/plugin-registry-generator/node_modules/simple-git/src/lib/runners/git-executor-chain.js:5:58)
Unable to start GitError: fatal: not a git repository (or any parent up to mount point /home)
Stopping at filesystem boundary (GIT_DISCOVERY_ACROSS_FILESYSTEM not set).

    at Object.action (/home/vsvydenko/.npm/_npx/292135/lib/node_modules/@eclipse-che/plugin-registry-generator/node_modules/simple-git/src/lib/plugins/error-detection.plugin.js:30:33)
    at PluginStore.exec (/home/vsvydenko/.npm/_npx/292135/lib/node_modules/@eclipse-che/plugin-registry-generator/node_modules/simple-git/src/lib/plugins/plugin-store.js:21:33)
    at /home/vsvydenko/.npm/_npx/292135/lib/node_modules/@eclipse-che/plugin-registry-generator/node_modules/simple-git/src/lib/runners/git-executor-chain.js:95:45
    at new Promise (<anonymous>)
    at GitExecutorChain.handleTaskData (/home/vsvydenko/.npm/_npx/292135/lib/node_modules/@eclipse-che/plugin-registry-generator/node_modules/simple-git/src/lib/runners/git-executor-chain.js:93:16)
    at GitExecutorChain.<anonymous> (/home/vsvydenko/.npm/_npx/292135/lib/node_modules/@eclipse-che/plugin-registry-generator/node_modules/simple-git/src/lib/runners/git-executor-chain.js:77:46)
    at Generator.next (<anonymous>)
    at fulfilled (/home/vsvydenko/.npm/_npx/292135/lib/node_modules/@eclipse-che/plugin-registry-generator/node_modules/simple-git/src/lib/runners/git-executor-chain.js:5:58) {
  task: {
    commands: [ 'rev-parse', '--show-toplevel' ],
    format: 'utf-8',
    parser: [Function: parser]
  }
}

Describe the solution you'd like

In case if parameter --skip-digest-generation was set as true we can skip code which requires simple-git library