* perf(git_status): tweak flags to omit extra info
`git status` can be prohibitively slow on some repos, so allow the
config to influence what flags are passed to git. For instance, if there
is no configured symbol for untracked files, tell git to omit them from
its output. This can easily result in a 2~10x speedup in many cases, but
requires the user to opt-in to hiding information from the prompt.
* docs(git_status): add ignore_submodules option
* git_status: added symbol for when local branch is up-to-date with upstream
* updated docs
* removed unused variable, moved location of config comment
* changed uptodate default to empty string, simplified and made safer
* added uptodate default line back into docstring
* fixed linting and formatting errors
* refactored uptodate to up_to_date, removed redundant else statement
