git_stash_pop: Add only updated files

This avoids adding untracked files to the repo. See here for a
description of the difference between `git add .` and `git add --update`:

https://stackoverflow.com/questions/572549/difference-between-git-add-a-and-git-add/572660#572660

thefuck/rules/git_stash_pop.py

@@ -11,7 +11,7 @@ def match(command):
 
 @git_support
 def get_new_command(command):
-    return shell.and_('git add .', 'git stash pop', 'git reset .')
+    return shell.and_('git add --update', 'git stash pop', 'git reset .')
 
 
 # make it come before the other applicable rules