Use `git checkout` instead of `git reset`

With this change, if the checkout fails, it will stop with an error.

The purpose is to avoid deleting working branch history when
developing gem.

lib/mruby/build/command.rb

@@ -279,7 +279,7 @@ module MRuby
 
   class Command::Git < Command
     attr_accessor :flags
-    attr_accessor :clone_options, :pull_options, :checkout_options, :reset_options
+    attr_accessor :clone_options, :pull_options, :checkout_options, :checkout_detach_options, :reset_options
 
     def initialize(build)
       super
@@ -288,6 +288,7 @@ module MRuby
       @clone_options = "clone %{flags} %{url} %{dir}"
       @pull_options = "--git-dir %{repo_dir}/.git --work-tree %{repo_dir} pull"
       @checkout_options = "--git-dir %{repo_dir}/.git --work-tree %{repo_dir} checkout %{checksum_hash}"
+      @checkout_detach_options = "--git-dir %{repo_dir}/.git --work-tree %{repo_dir} checkout --detach %{checksum_hash}"
       @reset_options = "--git-dir %{repo_dir}/.git --work-tree %{repo_dir} reset %{checksum_hash}"
     end
 
@@ -306,6 +307,11 @@ module MRuby
       _run checkout_options, { :checksum_hash => checksum_hash, :repo_dir => shellquote(dir) }
     end
 
+    def run_checkout_detach(dir, checksum_hash)
+      _pp "GIT CHECKOUT DETACH", dir, checksum_hash
+      _run checkout_detach_options, { :checksum_hash => checksum_hash, :repo_dir => shellquote(dir) }
+    end
+
     def run_reset_hard(dir, checksum_hash)
       _pp "GIT RESET", dir, checksum_hash
       _run reset_options, { :checksum_hash => checksum_hash, :repo_dir => shellquote(dir) }

lib/mruby/build/load_gems.rb

@@ -86,14 +86,13 @@ module MRuby
 
         if File.exist?(gemdir)
           if $pull_gems
-            git.run_pull gemdir, url
             # Jump to the top of the branch
-            git.run_checkout(gemdir, branch)
-            git.run_reset_hard gemdir, "origin/#{branch}"
+            git.run_checkout gemdir, branch
+            git.run_pull gemdir, url
           elsif params[:checksum_hash]
-            git.run_reset_hard(gemdir, params[:checksum_hash])
+            git.run_checkout_detach gemdir, params[:checksum_hash]
           elsif lock
-            git.run_reset_hard(gemdir, lock['commit'])
+            git.run_checkout_detach gemdir, lock['commit']
           end
         else
           options = [params[:options]] || []
@@ -105,9 +104,9 @@ module MRuby
 
           # Jump to the specified commit
           if params[:checksum_hash]
-            git.run_reset_hard gemdir, params[:checksum_hash]
+            git.run_checkout_detach gemdir, params[:checksum_hash]
           elsif lock
-            git.run_reset_hard gemdir, lock['commit']
+            git.run_checkout_detach gemdir, lock['commit']
           end
         end