Suggest brew link --overwrite --dry-run when relevant

This makes it easier to see which files would be overwritten by
`brew link --overwrite`

tests/rules/test_brew_link.py

@@ -0,0 +1,38 @@
+import pytest
+from tests.utils import Command
+from thefuck.rules.brew_link import get_new_command, match
+
+
+@pytest.fixture
+def stderr():
+    return ("Error: Could not symlink bin/gcp\n"
+			"Target /usr/local/bin/gcp\n"
+			"already exists. You may want to remove it:\n"
+			  "rm '/usr/local/bin/gcp'\n"
+			"\n"
+			"To force the link and overwrite all conflicting files:\n"
+			  "brew link --overwrite coreutils\n"
+			"\n"
+			"To list all files that would be deleted:\n"
+			  "brew link --overwrite --dry-run coreutils\n")
+
+
+@pytest.fixture
+def new_command(formula):
+    return 'brew link --overwrite --dry-run {}'.format(formula)
+
+
+@pytest.mark.parametrize('script', ['brew link coreutils', 'brew ln coreutils'])
+def test_match(stderr, script):
+    assert match(Command(script=script, stderr=stderr))
+
+
+@pytest.mark.parametrize('script', ['brew link coreutils'])
+def test_not_match(script):
+    stderr=''
+    assert not match(Command(script=script, stderr=stderr))
+
+
+@pytest.mark.parametrize('script, formula, ', [('brew link coreutils', 'coreutils')])
+def test_get_new_command(stderr, new_command, script, formula):
+    assert get_new_command(Command(script=script, stderr=stderr)) == new_command