tools: Add git diff.

- This is worth it afterall.
- Eris will get out of sync when the diffs are commited,
  but this is also true of git-blame and git-log.
- The user can refresh the old results with shift-r.

eris/tools.py

@@ -510,7 +510,7 @@ def make_tool_function(dependencies, command, url=None, success_status=None,
     return func
 
 
-elinks, git_blame = None, None  # For linters.
+elinks, git_diff, git_blame = None, None, None  # For linters.
 with importlib.resources.open_text(eris, "tools.toml") as tools_toml_file:
     tools_toml = toml.load(tools_toml_file)
 tools_for_extensions = tools_toml["tools_for_extensions"]
@@ -685,6 +685,7 @@ def _tools_for_extension():
 
 def tools_all():
     tools_ = set(generic_tools())
+    tools_.add(git_diff)
     tools_.add(git_blame)
     tools_.add(git_log)
     for tool_list in _tools_for_extension().values():
@@ -727,7 +728,7 @@ def is_tool_available(tool):
 
 
 def tools_for_path(path):
-    git_tools = [git_blame, git_log] if os.path.exists(".git") else []
+    git_tools = [git_diff, git_blame, git_log] if os.path.exists(".git") else []
     root, ext = splitext(path)
     extra_tools = [] if ext == "" else _tools_for_extension().get(ext[1:], [])
     tools = generic_tools() + git_tools + extra_tools

eris/tools.toml

@@ -111,6 +111,14 @@ tools_for_extensions = [
   error_status = "not_applicable"
   has_color = true
 
+[git_diff]
+  dependencies = ["git"]
+  url = "https://git-scm.com/docs/git-diff"
+  command = "git diff --word-diff=color --exit-code"
+  success_status = "normal"
+  error_status = "problem"
+  has_color = true
+
 [git_blame]
   dependencies = ["git"]
   url = "https://git-scm.com/docs/git-blame"