Change default of git_files to show_untracked=false

[kaddkaka]

Problem: git_files has an unexpected default of listing untracked files.
Suggestion: make the default of git_files not list untracked files.

:Telescope git_files currently lists all files in a git repo folder; the default is to also list untracked files. This is not the same default as git and other tools have. For example

• git ls-files lists all tracked files in the repo and a flag is needed to list untracked files as well.
• fzf.vim's :GFiles lists tracked files (has :GFiles? to list untracked files as well)

It is possible to list tracked files with lua require"telescope.builtin".git_files({ show_untracked = false }) but I think that the default behavior is unexpected. The first time I tried telescope I thought that git_files was broken because it acted just like find_files.

Related: There is already git_status which can be used to find new untracked files in the repo.

[Conni2461]

#842 was merged into dev branch. Please read #1938 if you want to know when it hits master.