Fix --dry-run flag

Summary:
Currently, `mononoke_newadmin` can be used with a command like
```
monoke_newadmin -- --prod hg-sync --repo-id 2103 last-processed --dry-run  --set 42
```

It won't dry run, but it will actually set the last processed ID in prod.

That is scary.

It is due to a surprising behaviour of clap that I reported upstream here: clap-rs/clap#4520.

Work around what is likely an upstream bug by adding an explicit `conflicts_with` configuration.

Reviewed By: YousefSalama, yancouto

Differential Revision: D41555183

fbshipit-source-id: 482046ade24b60961c3f3c19c309695ba4116b40

eden/mononoke/tools/admin/src/commands/hg_sync/last_processed.rs

@@ -32,7 +32,14 @@ pub struct HgSyncLastProcessedArgs {
 
     /// When skipping over blobimport entries, don't update the last processed
     /// entry, just print what would happen
-    #[clap(long, short = 'n', requires = "skip-blobimport")]
+    #[clap(
+        long,
+        short = 'n',
+        requires = "skip-blobimport",
+        conflicts_with = "set"
+    )] // You would think this "conflicts_with" wouldn't be required, but it is, due
+    // to a weird interaction with the `conflicts_with` on `skip_blobimport`.
+    // See upstream issue: https://github.com/clap-rs/clap/issues/4520
     dry_run: bool,
 }