feat: support custom format, plain sql and cluster dumps

README.md

@@ -83,10 +83,23 @@ ahab django make-command <app> <name>
 ## postgres
 
 ```bash
-ahab postgres dump <path>           # pg_dump, custom format
-ahab postgres import <path>         # drop, create, pg_restore
+ahab postgres dump <path>            # pg_dump, custom format
+ahab postgres dump -F plain <path>   # pg_dump, plain sql
+ahab postgres import <path>          # drop, create, then restore
 ```
 
+The format of a dump being imported is read from the file rather than its name.
+A custom format dump starts with `PGDMP` and a tar one with `toc.dat`, both of
+which go to `pg_restore`, as does a directory produced by `pg_dump -Fd`.
+Anything else is treated as sql and fed to `psql` with `ON_ERROR_STOP` and
+`--single-transaction`, so a bad file rolls back instead of half applying.
+A whole cluster dump from `pg_dumpall` is recognised by its header and handled
+differently again: it creates its own databases and carries role statements, so
+the database is dropped but not recreated, the dump goes to `psql` connected to
+`postgres`, and it runs without `ON_ERROR_STOP` because roles that already exist
+report errors that are expected. Gzipped dumps are decompressed on the way in,
+whichever of the three they hold.
+
 ## link
 
 `ahab link` moves untracked paths out of the repository into an out-of-repo