Fish, simplify functions, move n alias call

Simplifies the git_ family of functions to make fish_right_prompt more
readable. git_prompt function is added, which prints the relevant git
prompt information. The git_ family of functions is moved into the
git_prompt function, and fish_right_prompt calls git_prompt

Going forward I'm trying to have modules in charge of the data for
other modules, so I moved the nnn alias call from fish to n, and added a
handy "extraFunctions" config option to fish. Maintaining the fish
module's control of the functions, but allowing others to add stuff in

hmModules/apps/shells/fish/functions/git_prompt.fish

@@ -0,0 +1,69 @@
+function git_is_repo
+  test -d .git
+  or begin
+    set -l info (command git rev-parse --git-dir --is-bare-repository 2>/dev/null)
+    and test $info[2] = false
+  end
+end
+
+function git_is_worktree
+  git_is_repo
+  and test (command git rev-parse --is-inside-git-dir) = false
+end
+
+function git_is_dirty
+  git_is_worktree; and not command git diff --no-ext-diff --quiet --exit-code
+end
+
+function git_is_staged
+  git_is_repo; and begin
+    not command git diff --cached --no-ext-diff --quiet --exit-code
+  end
+end
+
+function git_branch_name
+  git_is_repo; and begin
+    command git symbolic-ref --short HEAD 2> /dev/null;
+      or command git show-ref --head -s --abbrev | head -n1 2> /dev/null
+  end
+end
+
+function git_is_touched
+  git_is_worktree; and begin
+    # The first checks for staged changes, the second for unstaged ones.
+    # We put them in this order because checking staged changes is *fast*.
+    not command git diff-index --cached --quiet HEAD -- >/dev/null 2>&1
+    or not command git diff --no-ext-diff --quiet --exit-code >/dev/null 2>&1
+  end
+end
+
+if git_is_repo
+    echo -n -s (set_color yellow) (git_branch_name) (set_color normal)
+    set -l git_meta ""
+    if test (command git ls-files --others --exclude-standard | wc -w 2> /dev/null) -gt 0
+        set git_meta "$git_meta?"
+    end
+    if test (command git rev-list --walk-reflogs --count refs/stash 2> /dev/null)
+        set git_meta "$git_meta\$"
+    end
+    if git_is_touched
+        git_is_dirty && set git_meta "$git_meta⨯"
+        git_is_staged && set git_meta "$git_meta●"
+    end
+    set -l commit_count (command git rev-list --count --left-right (git remote)/(git_branch_name)"...HEAD" 2> /dev/null)
+    if test $commit_count
+        set -l behind (echo $commit_count | cut -f 1)
+        set -l ahead (echo $commit_count | cut -f 2)
+        if test $behind -gt 0
+            set git_meta "$git_meta🠋"
+        end
+        if test $ahead -gt 0
+            set git_meta "$git_meta🠉"
+        end
+    end
+    if test $git_meta
+        echo -n -s (set_color red) " " $git_meta " " (set_color normal)
+    else
+        echo -n -s " "
+    end
+end