lingui extract <..files> overwrites obsolete flag

[nicolas-cherel]

When using lingui extract <some file> the result overwrite the obsolete flag of all the messages in the catalog to false. This is unexpected because lingui extract only adds obsolete: true flags and we probably don't want to overwrite this.

Eg lingui extract <file with obsolete translation> will produce the following patch that overwrite the obsolete flag

         20
       ]
     ],
-    "obsolete": true
+    "obsolete": false
   },

Also Eg lingui extract <file with changed translation> will produce the following change on listed files

+    ],
+    "obsolete": false

To Reproduce
do a lingui extract <some file> on a project with several files and see that "obsolete": false are added to catalog to translation of unlisted files, or have their "obsolete": true changed to "obsolete": false

Expected behavior
obsolete obsolete flag should be kept pristine when a list of file is supplied to the command line

Code fix proposal
Modify the line at https://github.com/lingui/js-lingui/blob/v4.11.1/packages/cli/src/api/catalog/mergeCatalog.ts#L52
from obsolete: !options.files, to ...(!options.files && { obsolete: true }),
in order to do nothing on obsolete flags for files not listed on command line.

[timofei-iatsenko]

You can prepare a PR with fix and tests we happily accept it.