When re importing originals unchanged a +active translation becomes obsolete

[petervanderdoes]

When you re import a original translation file, all translation marked as +active that were unchanged in the original translation file are set to -obsolete,

Fixes the reimporting problem

Works for me, needs blessing from nbachiyski

(In [657]) Do not leave unchanged active strings with their tempopary obsolete state

First, we used to mark all active strings as +obsolete, then mark all
current ones as +active.

Then, updating all strings on every POT import started causing performance
problems, so in [643] we stopped updating unchanged strings. This caused
unchanged active strings to be left in obsolete state.

The current approach is to leave all originals active, mark the new ones
as active, too and then, go through all removed and mark only them
as oboslete.

Historically, removed strings are fewer than added, that's why we chose to
remove them one by one, instead of marking only news ones.

Fixes #173, props petervanderdoes

[657] uses the wrong variable name in one spot, meaning that old strings won't get obsoleted. Patch attached.

(In [663]) Obsolete the right string when importing originals

We were using the variable $original, which isn't
the one from the loop, but has been used in previous
foreachs and thus have a value and we didn't get
errors.

The test needed to obsolete at least two originals, because
otherwise by chance we were obsoleting the right original.

Fixes #173, props nacin