amend_lines_on_revision_mismatch as DocumentSymbolsRequest option

Summary:
This flag will be disabled by default, but enabled on requests from CodeHub.

See diffs on top to see behavior, summary:

If a file changed between the requested revision and the (best) indexed revision, the symbol index will contain erronous line locations, which makes CodeHub render the wrong symbol information. This feature will enable a heuristic where symbols in unchanged lines should keep the correct locations.

Reviewed By: jjuliamolin, donsbot

Differential Revision: D78732535

fbshipit-source-id: 0a8450d3d2a7fccbb794f08c22c8e3fd678110d0

Author: RomanCastellarin

glean/glass/if/glass.thrift

@@ -164,6 +164,9 @@ struct RequestOptions {
 struct FeatureFlags {
   // include xlang? default is false
   3: optional bool include_xlang_refs;
+
+  // attempt to amend line numbers when the requested and served revisions mismatch
+  4: bool amend_lines_on_revision_mismatch = false;
 }
 
 // List symbols in a file. Symbols are spans of one or more tokens Glean has