Check whether we are inside the worktree (#651)

Some git commands require to be run from inside the worktree (as opposed
    to the git dir, although it's usually in .git). This commit adds
a function git_is_worktree to check this. It is used for the commands
that need the worktree instead of git_is_repo.

An alternative solution might have been to find the git worktree in the
parent of the git directory, but this doesn't work for all cases.
Generally it's impossible to detect the location of the worktree (plus
    it's not unique).
Co-authored-by: Pablo Aguiar <scorphus@gmail.com>

lib/git/git_is_dirty.fish

 function git_is_dirty -d "Check if there are changes to tracked files"
-  git_is_repo; and not command git diff --no-ext-diff --quiet --exit-code
+  git_is_worktree; and not command git diff --no-ext-diff --quiet --exit-code
 end

lib/git/git_is_touched.fish

 function git_is_touched -d "Check if repo has any changes"
-  git_is_repo; and begin
+  git_is_worktree; and begin
     # The first checks for staged changes, the second for unstaged ones.
     # We put them in this order because checking staged changes is *fast*.  
     not command git diff-index --cached --quiet HEAD -- >/dev/null 2>&1

lib/git/git_is_worktree.fish

+function git_is_worktree -d "Check if directory is inside the worktree of a repository"
+  git_is_repo
+  and test (command git rev-parse --is-inside-git-dir) = false
+end

lib/git/git_untracked.fish

 function git_untracked -d "Print list of untracked files"
-  git_is_repo; and begin
+  git_is_worktree; and begin
     command git ls-files --other --exclude-standard
   end
 end