Should ignore Cargo.toml files in .git

[stepancheg]

Got an error:

STDOUT ------------------------------------------------------------------------

STDERR ------------------------------------------------------------------------
Error: Failed to splice workspace

Caused by:
    Failed to read manifest at /Users/.../.git/sl/origbackups/.../Cargo.toml

ERROR: Analysis of target '//...' failed; build aborted: error evaluating module extension crate in @@rules_rust~//crate_universe:extensions.bzl

.git/sl is a directory created by Sapling. Rust rules should not look inside .git directory.

Adding .git to .bazelignore does not help.

[aldanor]

Related: #3425