fix unused variable warning

Summary:
Fixes this (which was polluting my `arc rust-check` output):

```
warning: unused variable: `time`
   --> fbcode/eden/scm/saplingnative/bindings/modules/pymetalog/src/lib.rs:119:38
    |
119 |     def commit(&self, message: &str, time: Option<u64> = None, pending: bool = false) -> PyResult<Bytes> {
    |                                      ^^^^
    |
help: `time` is captured in macro and introduced a unused variable
   --> third-party/rust/vendor/cpython-0.7.2/src/py_class/py_class.rs:478:1
    |
    = note: in this expansion of `py_class!` (#1)
   ::: third-party/rust/vendor/cpython-0.7.2/src/py_class/py_class.rs:537:9
    |
    = note: in this macro invocation (#2)
   ::: third-party/rust/vendor/cpython-0.7.2/src/py_class/py_class.rs:543:1
    |
    = note: in this expansion of `$crate::py_class_impl_item!` (#18)
   --> third-party/rust/vendor/cpython-0.7.2/src/py_class/py_class_impl3.rs:30:1
    |
    = note: in this expansion of `$crate::py_class_impl!` (#2)
    |
    = note: in this expansion of `$crate::py_class_impl!` (#3)
    |
    = note: in this expansion of `$crate::py_class_impl!` (#4)
    |
    = note: in this expansion of `$crate::py_class_impl!` (#5)
    |
    = note: in this expansion of `$crate::py_class_impl!` (#6)
    |
    = note: in this expansion of `$crate::py_class_impl!` (#7)
    |
    = note: in this expansion of `$crate::py_class_impl!` (#8)
    |
    = note: in this expansion of `$crate::py_class_impl!` (#9)
    |
    = note: in this expansion of `$crate::py_class_impl!` (#10)
    |
    = note: in this expansion of `$crate::py_class_impl!` (#11)
    |
    = note: in this expansion of `$crate::py_class_impl!` (#12)
    |
    = note: in this expansion of `$crate::py_class_impl!` (#13)
   ::: third-party/rust/vendor/cpython-0.7.2/src/py_class/py_class_impl3.rs:249:5
    |
    = note: in this macro invocation (#3)
    |
    = note: in this macro invocation (#4)
   ::: third-party/rust/vendor/cpython-0.7.2/src/py_class/py_class_impl3.rs:2392:5
    |
    = note: in this macro invocation (#5)
   ::: third-party/rust/vendor/cpython-0.7.2/src/py_class/py_class_impl3.rs:2970:5
    |
    = note: in this macro invocation (#7)
    |
    = note: in this macro invocation (#13)
   ::: third-party/rust/vendor/cpython-0.7.2/src/py_class/py_class_impl3.rs:2999:13
    |
    = note: in this macro invocation (#14)
   ::: third-party/rust/vendor/cpython-0.7.2/src/py_class/py_class_impl3.rs:3005:5
    |
    = note: in this macro invocation (#8)
    |
    = note: in this macro invocation (#11)
    |
    = note: in this macro invocation (#12)
   ::: third-party/rust/vendor/cpython-0.7.2/src/py_class/py_class_impl3.rs:3120:5
    |
    = note: in this macro invocation (#6)
    |
    = note: in this macro invocation (#9)
    |
    = note: in this macro invocation (#10)
   --> third-party/rust/vendor/cpython-0.7.2/src/argparse.rs:196:1
    |
    = note: in this expansion of `$crate::py_argparse_parse_plist_impl!` (#14)
    |
    = note: in this expansion of `$crate::py_argparse_parse_plist_impl!` (#15)
    |
    = note: in this expansion of `$crate::py_argparse_parse_plist_impl!` (#16)
    |
    = note: in this expansion of `$crate::py_argparse_parse_plist_impl!` (#17)
   ::: third-party/rust/vendor/cpython-0.7.2/src/argparse.rs:201:9
    |
    = note: in this macro invocation (#18)
   ::: third-party/rust/vendor/cpython-0.7.2/src/argparse.rs:271:9
    |
    = note: in this macro invocation (#15)
   ::: third-party/rust/vendor/cpython-0.7.2/src/argparse.rs:331:9
    |
    = note: in this macro invocation (#16)
    |
    = note: in this macro invocation (#17)
    |
   ::: fbcode/eden/scm/saplingnative/bindings/modules/pymetalog/src/lib.rs:36:1
    |
36  | /   py_class!(pub class metalog |py| {
37  | |       data log: Arc<RwLock<MetaLog>>;
38  | |       data fspath: String;
...   |
226 | |       }
227 | |   });
    | |____- in this macro invocation (#1)
    = note: `#[warn(unused_variables)]` on by default

```

Reviewed By: quark-zju

Differential Revision: D65218833

fbshipit-source-id: fa69c1a24a32b7eff857070528f6337ec0b3711c

eden/scm/saplingnative/bindings/modules/pymetalog/src/lib.rs

@@ -116,7 +116,7 @@ py_class!(pub class metalog |py| {
     }
 
     /// Write pending data to disk. Raise if race condition is detected.
-    def commit(&self, message: &str, time: Option<u64> = None, pending: bool = false) -> PyResult<Bytes> {
+    def commit(&self, message: &str, _time: Option<u64> = None, pending: bool = false) -> PyResult<Bytes> {
         let mut opts = CommitOptions::default();
         opts.detached = pending;
         opts.message = message;