repart: Clarify and test that --copy-from= argument respects grain size

The --copy-from= argument currently is documented as "copied partitions will have
the same size". This doesn't hold true in the case where a different grain-size is
passed to repart. Because `partition_min/max_size()` currently do rounding, the
size is implicitly rounded to grain size, and therefore partitions are enlarged
to align to grain size whenever possible.

Clarify this behavior and change the manpage, and also add a test for it.

test/units/TEST-58-REPART.sh

@@ -493,6 +493,40 @@ EOF
     assert_in "$imgs/copy_to2 : start=       51200, size=       10240, type=${xbootldr_guid}," "$output"
 }
 
+testcase_copy_from_respects_new_grain() {
+    local defs imgs output
+
+    defs="$(mktemp --directory "/tmp/test-repart.defs.XXXXXXXXXX")"
+    imgs="$(mktemp --directory "/var/tmp/test-repart.imgs.XXXXXXXXXX")"
+    # shellcheck disable=SC2064
+    trap "rm -rf '$defs' '$imgs'" RETURN
+    chmod 0755 "$defs"
+
+    truncate -s 80MiB "$imgs/copy_from"
+    sfdisk "$imgs/copy_from" <<EOF
+label: gpt
+
+size=20480, type=${root_guid}, uuid=837c3d67-21b3-478e-be82-7e7f83bf96d3
+size=10240, type=${xbootldr_guid}, uuid=4985c03e-eecb-4fe0-9f65-3f6345782214
+EOF
+
+    truncate -s 100MiB "$imgs/copy_to"
+    output=$(systemd-repart --offline="$OFFLINE" \
+                            --empty=allow \
+                            --definitions="$defs" \
+                            --seed="$seed" \
+                            --dry-run=no \
+                            --json=pretty \
+                            --copy-from="$imgs/copy_from" \
+                            --grain-size=4194304 \
+                            "$imgs/copy_to")
+
+    output=$(sfdisk --dump "$imgs/copy_to")
+
+    assert_in "$imgs/copy_to1 : start=        8192, size=       24576, type=${root_guid}," "$output"
+    assert_in "$imgs/copy_to2 : start=       32768, size=       16384, type=${xbootldr_guid}," "$output"
+}
+
 testcase_dropin() {
     local defs imgs output