repart: Add --copy-from option

--copy-from synthesizes partition definitions from the given image
which are then applied to the repart algorithm. In its most basic
form, this allows copying an image to another device but it can
also be combined with --definitions to copy + add partitions in the
same call to repart.

test/units/testsuite-58.sh

@@ -160,6 +160,27 @@ last-lba: 2097118
 $imgs/zzz1 : start=        2048, size=     1775576, type=933AC7E1-2EB4-4F13-B844-0E14E2AEF915, uuid=4980595D-D74A-483A-AA9E-9903879A0EE5, name=\"home-first\", attrs=\"GUID:59\"
 $imgs/zzz2 : start=     1777624, size=      131072, type=0657FD6D-A4AB-43C4-84E5-0933C84B4F4F, uuid=78C92DB8-3D2B-4823-B0DC-792B78F66F1E, name=\"swap\""
 
+    systemd-repart --offline="$OFFLINE" \
+                   --empty=create \
+                   --size=1G \
+                   --dry-run=no \
+                   --seed="$seed" \
+                   --copy-from="$imgs/zzz" \
+                   "$imgs/copy"
+
+    output=$(sfdisk -d "$imgs/copy" | grep -v -e 'sector-size' -e '^$')
+
+    assert_eq "$output" "label: gpt
+label-id: 1D2CE291-7CCE-4F7D-BC83-FDB49AD74EBD
+device: $imgs/copy
+unit: sectors
+first-lba: 2048
+last-lba: 2097118
+$imgs/copy1 : start=        2048, size=     1775576, type=933AC7E1-2EB4-4F13-B844-0E14E2AEF915, uuid=4980595D-D74A-483A-AA9E-9903879A0EE5, name=\"home-first\", attrs=\"GUID:59\"
+$imgs/copy2 : start=     1777624, size=      131072, type=0657FD6D-A4AB-43C4-84E5-0933C84B4F4F, uuid=78C92DB8-3D2B-4823-B0DC-792B78F66F1E, name=\"swap\""
+
+    rm "$imgs/copy" # Save disk space
+
     systemd-repart --offline="$OFFLINE" \
                    --definitions="$defs" \
                    --dry-run=no \