Implement :checksum_hash mrbgem option.

This allows to use a mgem with a specific checksum hash.
The following modification were necessary to implement this
function:
  - add run_checkout build command to use 'git checkout'
  - skip shallow copy in case checksum_hash is used
  - make 'master' the default branch if branch isn't defined

build_config.rb

@@ -16,7 +16,7 @@ MRuby::Build.new do |conf|
   #   g.cc.flags << '-g' # append cflags in this gem
   # end
   # conf.gem 'examples/mrbgems/c_and_ruby_extension_example'
-  # conf.gem :github => 'masuidrive/mrbgems-example', :branch => 'master'
+  # conf.gem :github => 'masuidrive/mrbgems-example', :checksum_hash => '76518e8aecd131d047378448ac8055fa29d974a9'
   # conf.gem :git => 'git@github.com:masuidrive/mrbgems-example.git', :branch => 'master', :options => '-v'
 
   # include the default GEMs

tasks/mruby_build_commands.rake

@@ -228,14 +228,15 @@ module MRuby
 
   class Command::Git < Command
     attr_accessor :flags
-    attr_accessor :clone_options, :pull_options
+    attr_accessor :clone_options, :pull_options, :checkout_options
 
     def initialize(build)
       super
       @command = 'git'
-      @flags = %w[--depth 1]
+      @flags = %w[]
       @clone_options = "clone %{flags} %{url} %{dir}"
       @pull_options = "pull"
+      @checkout_options = "checkout %{checksum_hash}"
     end
 
     def run_clone(dir, url, _flags = [])
@@ -250,6 +251,14 @@ module MRuby
       _run pull_options
       Dir.chdir root
     end
+
+    def run_checkout(dir, checksum_hash)
+      root = Dir.pwd
+      Dir.chdir dir
+      _pp "GIT CHECKOUT", checksum_hash
+      _run checkout_options, { :checksum_hash => checksum_hash }
+      Dir.chdir root
+    end
   end
 
   class Command::Mrbc < Command

tasks/mruby_build_gem.rake

@@ -59,6 +59,9 @@ module MRuby
         url = params[:git]
         gemdir = "#{gem_clone_dir}/#{url.match(/([-\w]+)(\.[-\w]+|)$/).to_a[1]}"
 
+        # by default the 'master' branch is used
+        branch = params[:branch] ? params[:branch] : 'master'
+
         if File.exist?(gemdir)
           if $pull_gems
             git.run_pull gemdir, url
@@ -67,10 +70,19 @@ module MRuby
           end
         else
           options = [params[:options]] || []
-          options << "--branch \"#{params[:branch]}\"" if params[:branch]
+          options << "--branch \"#{branch}\""
+          options << "--depth 1" unless params[:checksum_hash]
           FileUtils.mkdir_p "#{gem_clone_dir}"
           git.run_clone gemdir, url, options
         end
+
+        if params[:checksum_hash]
+          # Jump to the specified commit
+          git.run_checkout gemdir, params[:checksum_hash]
+        else
+          # Jump to the top of the branch
+          git.run_checkout gemdir, branch
+        end
       else
         fail "unknown gem option #{params}"
       end