Add git_rm_staged rule for removing locally staged changes

It would be nice if `thefuck` could help me `git rm` a file I had
already staged. This rule, adapted from `git_rm_local_modifications`,
does that.

thefuck/rules/git_rm_staged.py

@@ -0,0 +1,19 @@
+from thefuck.specific.git import git_support
+
+
+@git_support
+def match(command):
+    return (' rm ' in command.script and
+            'error: the following file has changes staged in the index' in command.stderr and
+            'use --cached to keep the file, or -f to force removal' in command.stderr)
+
+
+@git_support
+def get_new_command(command):
+    command_parts = command.script_parts[:]
+    index = command_parts.index('rm') + 1
+    command_parts.insert(index, '--cached')
+    command_list = [u' '.join(command_parts)]
+    command_parts[index] = '-f'
+    command_list.append(u' '.join(command_parts))
+    return command_list