Remove Xgit.Util.FileSnapshot. (#286)

This need is better served by Xgit.Util.ObservedFile.

This allows us to remove the dependency on ConCache.

lib/xgit.ex

@@ -9,13 +9,6 @@ defmodule Xgit do
   """
   @impl true
   def start(_type, _args) do
-    children = [
-      {ConCache,
-       name: :xgit_file_snapshot,
-       ttl_check_interval: :timer.seconds(1),
-       global_ttl: :timer.seconds(5)}
-    ]
-
-    Supervisor.start_link(children, strategy: :one_for_one)
+    Supervisor.start_link([], strategy: :one_for_one)
   end
 end

mix.exs

@@ -26,7 +26,6 @@ defmodule Xgit.MixProject do
   defp deps do
     [
       {:benchee, "~> 1.0", only: :dev},
-      {:con_cache, "~> 0.13"},
       {:credo, "~> 1.1", only: [:dev, :test]},
       {:dialyxir, "~> 1.0.0-rc.6", only: :dev, runtime: false},
       {:excoveralls, "~> 0.11", only: :test},

mix.lock

@@ -2,7 +2,6 @@
   "benchee": {:hex, :benchee, "1.0.1", "66b211f9bfd84bd97e6d1beaddf8fc2312aaabe192f776e8931cb0c16f53a521", [:mix], [{:deep_merge, "~> 1.0", [hex: :deep_merge, optional: false]}]},
   "bunt": {:hex, :bunt, "0.2.0", "951c6e801e8b1d2cbe58ebbd3e616a869061ddadcc4863d0a2182541acae9a38", [:mix], []},
   "certifi": {:hex, :certifi, "2.5.1", "867ce347f7c7d78563450a18a6a28a8090331e77fa02380b4a21962a65d36ee5", [:rebar3], [{:parse_trans, "~>3.3", [hex: :parse_trans, optional: false]}]},
-  "con_cache": {:hex, :con_cache, "0.14.0", "863acb90fa08017be3129074993af944cf7a4b6c3ee7c06c5cd0ed6b94fbc223", [:mix], []},
   "credo": {:hex, :credo, "1.1.5", "caec7a3cadd2e58609d7ee25b3931b129e739e070539ad1a0cd7efeeb47014f4", [:mix], [{:bunt, "~> 0.2.0", [hex: :bunt, repo: "hexpm", optional: false]}, {:jason, "~> 1.0", [hex: :jason, repo: "hexpm", optional: false]}], "hexpm"},
   "deep_merge": {:hex, :deep_merge, "1.0.0", "b4aa1a0d1acac393bdf38b2291af38cb1d4a52806cf7a4906f718e1feb5ee961", [:mix], []},
   "dialyxir": {:hex, :dialyxir, "1.0.0-rc.7", "6287f8f2cb45df8584317a4be1075b8c9b8a69de8eeb82b4d9e6c761cf2664cd", [:mix], [{:erlex, ">= 0.2.5", [hex: :erlex, repo: "hexpm", optional: false]}], "hexpm"},