Add spec tests for Git blob include processor

spec/fixtures/HEAD-LICENSE

@@ -0,0 +1,19 @@
+Copyright (c) 2023 Gabriel Arazas <foodogsquared@foodogsquared.one>
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in
+all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+THE SOFTWARE.

spec/fixtures/v1.0.0-shell.nix

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+{ pkgs ? import <nixpkgs> { }, ruby-nix }:
+
+with pkgs;
+
+let
+  gems = ruby-nix.lib pkgs {
+    name = "asciidoctor-foodogsquared-extensions";
+    ruby = ruby_3_1;
+    gemset = ./gemset.nix;
+  };
+in
+mkShell {
+  buildInputs = [
+    gems.env
+    gems.ruby
+  ];
+
+  packages = [
+    bundix
+
+    # Formatters
+    nixpkgs-fmt
+
+    # Language servers
+    rnix-lsp
+  ];
+}

spec/git_blob_include_processor_spec.rb

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+# frozen_string_literal: true
+
+require 'open-uri'
+
+describe GitBlobIncludeProcessor do
+  it 'should find the LICENSE from the head commit of this Git repo' do
+    input = <<~INPUT
+      [source]
+      ----
+      include::git:HEAD[path=LICENSE]
+      ----
+    INPUT
+
+    expected = File.read(fixtures_file('HEAD-LICENSE'))
+
+    actual = (Asciidoctor.convert input).tr_s '\n', '\n'
+    (expect actual).to include expected.chomp
+  end
+
+  it 'should find the README from a certain commit' do
+    input = <<~INPUT
+      [source]
+      ----
+      include::git:v1.0.0[path=shell.nix]
+      ----
+    INPUT
+
+    expected = File.read(fixtures_file('v1.0.0-shell.nix'))
+
+    actual = (Asciidoctor.convert input).tr_s '\n', '\n'
+    (expect actual).to include expected.chomp
+  end
+
+  it 'should not find the non-existent commit from this repo' do
+    input = <<~INPUT
+      [source]
+      ----
+      include::git:00000000-0000-0000-0000-000000000000[path=README.adoc]
+      ----
+    INPUT
+
+    expected = <<~RESULT
+      Unresolved directive for 'git:00000000-0000-0000-0000-000000000000' with the following error:
+      revspec '00000000-0000-0000-0000-000000000000' not found
+    RESULT
+
+    actual = (Asciidoctor.convert input).tr_s '\n', '\n'
+    (expect actual).to include expected.chomp
+  end
+
+  it 'should not find the non-existent repo' do
+    input = <<~INPUT
+      :gitrepo: ../very-homeless
+
+      [source]
+      -----
+      include::git:HEAD[path=README.adoc]
+      ----
+    INPUT
+
+    expect { Asciidoctor.convert input }.to raise_error StandardError
+  end
+end