Git: Archive support

This allows users to create archive files from local repository.

Tools > Git > Local Repository > Archive will create <reponame>-
HEAD.tar.gz

Tools > Git > Actions on Commits... you can choose a commit to archive

Change-Id: I09bbbdefe532237e9065d1ca3c21910a036ea149
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: André Hartmann <aha_1980@gmx.de>
Reviewed-by: Tasuku Suzuki <tasuku.suzuki@qbc.io>

src/plugins/git/changeselectiondialog.h

@@ -42,6 +42,7 @@ namespace Internal {
 
 enum ChangeCommand {
     NoCommand,
+    Archive,
     Checkout,
     CherryPick,
     Revert,
@@ -68,6 +69,7 @@ private:
     void recalculateCompletion();
     void recalculateDetails();
     void changeTextChanged(const QString &text);
+    void acceptArchive();
     void acceptCheckout();
     void acceptCherryPick();
     void acceptRevert();