Giterminism refinements: full git-submodules support, better custom project dir support

[giterminism] Improve feedback if a related submodule is not clean or has changes

• Ignore user work tree submodule directory if a submodule ".git" directory not found

• Return a detailed error if a work tree submodule directory is not clean:

  the submodule "<submodule path>" is not clean and must be committed. Do not forget to push the current commit to the submodule remote If this commit exists only locally
  
  Details:
      commit:                 "0000000000000000000000000000000000000000"
      currentWorktreeCommit:  "d9243a9a45b10e6b7d985322b8108897626a25a6"
      expectedWorktreeCommit: "d9243a9a45b10e6b7d985322b8108897626a25a6"
  
  You might also be interested in developer mode (activated with --dev option) that allows you to work with staged changes without doing redundant commits. Just use "git add <file>..." to include the changes that should be used.
  

• Return a detailed error if a work tree submodule directory has changes:

  the submodule "<submodule path>" has modified files and these changes must be committed (do not forget to push new changes to the submodule remote) or discarded:
  
      - file1
      - file2
  

[giterminism] Support for configuration files outside the project directory

Internals:

• [tests] Giterminism Suite: test custom project directory
• [giterminism] Refactor symlink check code

Docs

[doc] Add Rails guide

Internals

[tests] Fix daily tests, not compiling and failing tests, refactor tests precompile

Installation

Linux amd64

Darwin amd64

Windows amd64