magit-fetch-popup: add magit-submodule fetch
refs: make unmerged-commits counts optional

In `magit-insert-branch-1' only calculate the number of unmerged files
if FORMAT (`magit-local-branch-format' or `magit-remote-branch-format')
contains "%c".  In `magit-insert-tags' only do so if `magit-tags-format'
contains "%c".  Add new variable `magit-refs-indent-cherry-lines'.

Calculating the number of unmerged commits is less demanding than
calculating cherry commits, but it's still to slow in big repositories.

To avoid doing so use something like:

(setq magit-local-branch-format "%C %-25n %U%m\n"
      magit-remote-branch-format "  %-25n %m\n"
      magit-tags-format "  %-25n %m\n"
      magit-refs-indent-cherry-lines 0)

Also see #1674.