Only merge acquire items with the same meta key

Since the introduction of by-hash, two differently named
files might have the same real URL. In our case, the files
icons-64x64.tar.gz and icons-128x128.tar.gz of empty tarballs.

APT would try to merge them and end with weird errors because
it completed the first download and enters the second stage for
decompressing and verifying. After that it would queue a new item
to copy the original file to the location, but that copy item would
be in the wrong stage, causing it to use the hashes for the
decompressed item.

Closes: #838441
(cherry picked from commit 7b78e8bef1fc9de22d826db1db9df25f97d3710c)
(cherry picked from commit d2749c845954fc1ea38133b050ee49d6f6544235)

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