Fix git stash pop with local changes

When there are local changes to a file, and a git stash is popped that
contains other changes to that same file, git fails as follows:

    $ git stash pop
    error: Your local changes to the following files would be overwritten by merge:
            src/index.js
    Please commit your changes or stash them before you merge.
    Aborting
    $

This change adds a rule that corrects this problem as suggested [here]:

    $ git stash pop
    error: Your local changes to the following files would be overwritten by merge:
            src/index.js
    Please commit your changes or stash them before you merge.
    Aborting
    $ fuck
    git add . && git stash pop && git reset . [enter/↑/↓/ctrl+c]
    Auto-merging src/index.js
    On branch flow
    Changes to be committed:
      (use "git reset HEAD <file>..." to unstage)

            modified:   src/index.js

    Changes not staged for commit:
      (use "git add <file>..." to update what will be committed)
      (use "git checkout -- <file>..." to discard changes in working directory)

            modified:   src/index.js

    Dropped refs/stash@{0} (f94776d484c4278997ac6837a7b138b9b9cdead1)
    Unstaged changes after reset:
    M        src/index.js
    $

[here]: https://stackoverflow.com/questions/15126463/how-do-i-merge-local-modifications-with-a-git-stash-without-an-extra-commit/15126489#15126489

tests/rules/test_git_stash_pop.py

@@ -0,0 +1,18 @@
+import pytest
+from thefuck.rules.git_stash_pop import match, get_new_command
+from tests.utils import Command
+
+
+@pytest.fixture
+def stderr():
+    return '''error: Your local changes to the following files would be overwritten by merge:'''
+
+
+def test_match(stderr):
+    assert match(Command('git stash pop', stderr=stderr))
+    assert not match(Command('git stash'))
+
+
+def test_get_new_command(stderr):
+    assert get_new_command(Command('git stash pop', stderr=stderr)) \
+           == "git add . && git stash pop && git reset ."