feat(git-summary): add stashed count alongside uncommitted

[pierreay]

Problem

In git helpers, when querying the status of a repository, it is often desired or expected to see if there is one or more stash(es) inside a repository alongside uncommitted changes or files (or at least, to have the choice to do so).

Solution

Add a new column (after uncommitted, before author for long output style, before branch name for tabular or oneline output style) counting the number of stash(es) inside the repository.

[pierreay]

TODO if necessary

If this PR is of interest for the maintainers, I can update the documentation/manpage to explicitly state that we check for stashes and to update the examples with the new output.

[hyperupcall]

Sounds good to me. @spacewander what do you think?