isl-server: update PLAINEXCEPT to bypass sniff limitation

Summary: See the previous diff D54221105 for context.

Reviewed By: evangrayk

Differential Revision: D54221104

fbshipit-source-id: b012e81f54302427eceda1e2e34f300f07793479

addons/isl-server/src/commands.ts

@@ -194,7 +194,9 @@ export function getExecParams(
     SL_ENCODING: 'UTF-8',
     // override any custom aliases a user has defined.
     SL_AUTOMATION: 'true',
-    SL_AUTOMATION_EXCEPT: 'phrevset', // allow looking up diff numbers even in plain mode
+    // allow looking up diff numbers even in plain mode.
+    // allow constructing the `.git/sl` repo regardless of the identity.
+    SL_AUTOMATION_EXCEPT: 'phrevset,sniff',
     // Prevent user-specified merge tools from attempting to
     // open interactive editors.
     HGMERGE: ':merge3',