Do not reset the source directory if there is modification.

We simply do a `git merge --ff-only` instead of a `git reset`. This way:

- No change on the local repository
    => update to `remote/master`
- If you switch branch and local branch is behind `remote/master`
    => fast forward to `remote/master`
- If you switch branch and branches diverge
    => No update of the directory
- On branch master (or a branch behind `origin/master`) with compatible
  change in the working tree.
    =>  update of the directory
- On branch master (or a branch behind `origin/master`) but incompatible
  change in the working tree.
    =>  No update.

This way, sources are keep up-to-date as far as possible.

kiwixbuild/__init__.py

@@ -48,6 +48,10 @@ def parse_args():
                           help=("Specify the architecture to build for ios application/libraries.\n"
                                 "Can be specified several times to build for several architectures.\n"
                                 "If not specified, all architectures will be build."))
+    subgroup.add_argument('--fast-clone', action='store_true',
+                          help=("Do not clone the whole repository.\n"
+                                "This is useful for one shot build but it is not recommended if you want "
+                                "to develop with the cloned sources."))
     options = parser.parse_args()
 
     if not options.android_arch: