pkgdev manifest --if-modified removes checksums for "old" files

[mgorny]

When called with --if-modified, pkgdev includes Manifest entries only for distfiles referenced by modified ebuilds and discards all other entries.

$ git checkout 927204470935f67016aeea6f3c285794def9f0a9
$ cp sys-devel/llvm/llvm-14.0.{4,5}.ebuild
$ git add sys-devel/llvm/llvm-14.0.5.ebuild
$ pkgdev manifest --if-modified
 * generating manifest: sys-devel/llvm::gentoo
$ git diff | cat
diff --git a/sys-devel/llvm/Manifest b/sys-devel/llvm/Manifest
index b449fc4d4683..a94fedfa1265 100644
--- a/sys-devel/llvm/Manifest
+++ b/sys-devel/llvm/Manifest
@@ -1,16 +1,3 @@
-DIST llvm-12.0.1-manpages.tar.bz2 191582 BLAKE2B a049cd305c257875d47e92d703c51aee8225854f999eddbd530c7a978704a981b05bc0819ba31a2f2a218d3569b078f0deb01b0eeb385966253b54a0fdc090d1 SHA512 9931a57f843709aed59193524fe980af695e4e23c8f53ec3cff308677ba4fc520ca58032b50b68440c7762305855317d9a705fe5ccbb543fb104f4deb5d10ea5
-DIST llvm-13.0.1-manpages.tar.bz2 202187 BLAKE2B 32a761426dbe4c3c355d30660254305cd88b6d513faee04c78e370cea9d38761108a9be4c4b4ac39b6a74400af1f9d37ffd11ab05acd7340175c8d66845ae7d9 SHA512 9495e1daf588540fdb8b9ee905a9b6be4693bd70b7f42c8e8a0d667242fc24dc853c45918d3bc09d02f0761ad61541bc9749c1498e312da129abdab638a0d572
-DIST llvm-14.0.1-manpages.tar.bz2 86273 BLAKE2B 174b412568565bdd4c3db34b1b2f2482a18ca45a0a6269ca381fe5c77da21b85dafabad29551b262d593006e9b68e6f7fdf43c2ec626204970d1280dd87a5f3d SHA512 49c58094eebecfadff97d62b4e6b0129e07c52ad7fe3ea4ce5c955e6fc9c73ac443b6a6e0f8322cf25e1d88b059c4231c4798d8da04340a46f6f84fb1b386f29
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