Vcs prompt: Break if vcs isn't installed

Prevents an annoying error message.
Fixes #2363.

share/functions/__fish_git_prompt.fish

@@ -346,6 +346,11 @@ function __fish_git_prompt_show_upstream --description "Helper function for __fi
 end
 
 function __fish_git_prompt --description "Prompt function for Git"
+	# If git isn't installed, there's nothing we can do
+	# Return 1 so the calling prompt can deal with it
+	if not command -s git >/dev/null
+		return 1
+	end
 	set -l repo_info (command git rev-parse --git-dir --is-inside-git-dir --is-bare-repository --is-inside-work-tree --short HEAD ^/dev/null)
 	test -n "$repo_info"; or return
 

share/functions/__fish_hg_prompt.fish

@@ -21,6 +21,11 @@ set -g fish_prompt_hg_status_unmerged '!'
 set -g fish_prompt_hg_status_order added modified copied deleted untracked unmerged
 
 function __fish_hg_prompt --description 'Write out the hg prompt'
+	# If hg isn't installed, there's nothing we can do
+	# Return 1 so the calling prompt can deal with it
+	if not command -s hg >/dev/null
+		return 1
+	end
     set -l branch (hg branch ^/dev/null)
     if test -z $branch
         return

share/functions/__terlar_git_prompt.fish

@@ -21,6 +21,11 @@ set -g fish_prompt_git_status_unmerged '!'
 set -g fish_prompt_git_status_order added modified renamed copied deleted untracked unmerged
 
 function __terlar_git_prompt --description 'Write out the git prompt'
+  # If git isn't installed, there's nothing we can do
+  # Return 1 so the calling prompt can deal with it
+  if not command -s git >/dev/null
+    return 1
+  end
   set -l branch (git rev-parse --abbrev-ref HEAD ^/dev/null)
   if test -z $branch
     return