made the pack completely portable and wrote relevent bat files to go with it

gitportable/mingw64/share/git/completion/meson.build

@@ -0,0 +1,16 @@
+foreach script : [
+  'git-completion.bash',
+  'git-completion.tcsh',
+  'git-completion.zsh',
+  'git-prompt.sh'
+]
+  if meson.version().version_compare('>=1.3.0')
+    test_dependencies += fs.copyfile(script)
+  else
+    configure_file(
+      input: script,
+      output: script,
+      copy: true,
+    )
+  endif
+endforeach