tests: fix mtime flakiness in git gc tests

Apparently, these gc() invocations rely on that the previous "git gc" packed
all refs so there are no loose refs to compare mtimes. If there were remaining
(or new?) loose refs, mtime comparison could fail. Let's add +1sec to
effectively turn off the keep_newer option.

Fixes #3537

lib/tests/test_git_backend.rs

@@ -15,7 +15,7 @@
 use std::collections::HashSet;
 use std::process::Command;
 use std::sync::Arc;
-use std::time::SystemTime;
+use std::time::{Duration, SystemTime};
 
 use jj_lib::backend::CommitId;
 use jj_lib::git_backend::GitBackend;
@@ -126,9 +126,12 @@ fn test_gc() {
         },
     );
 
+    // Don't rely on the exact system time because file modification time might
+    // have lower precision for example.
+    let now = || SystemTime::now() + Duration::from_secs(1);
+
     // All reachable: redundant no-gc refs will be removed
-    let now = SystemTime::now();
-    repo.store().gc(repo.index(), now).unwrap();
+    repo.store().gc(repo.index(), now()).unwrap();
     assert_eq!(
         collect_no_gc_refs(&git_repo),
         hashset! {
@@ -147,7 +150,7 @@ fn test_gc() {
     mut_index.add_commit(&commit_e);
     mut_index.add_commit(&commit_f);
     mut_index.add_commit(&commit_h);
-    repo.store().gc(mut_index.as_index(), now).unwrap();
+    repo.store().gc(mut_index.as_index(), now()).unwrap();
     assert_eq!(
         collect_no_gc_refs(&git_repo),
         hashset! {
@@ -163,7 +166,7 @@ fn test_gc() {
     mut_index.add_commit(&commit_b);
     mut_index.add_commit(&commit_c);
     mut_index.add_commit(&commit_f);
-    repo.store().gc(mut_index.as_index(), now).unwrap();
+    repo.store().gc(mut_index.as_index(), now()).unwrap();
     assert_eq!(
         collect_no_gc_refs(&git_repo),
         hashset! {
@@ -175,7 +178,7 @@ fn test_gc() {
     // B|C|F are no longer reachable
     let mut mut_index = base_index.start_modification();
     mut_index.add_commit(&commit_a);
-    repo.store().gc(mut_index.as_index(), now).unwrap();
+    repo.store().gc(mut_index.as_index(), now()).unwrap();
     assert_eq!(
         collect_no_gc_refs(&git_repo),
         hashset! {
@@ -184,6 +187,6 @@ fn test_gc() {
     );
 
     // All unreachable
-    repo.store().gc(base_index.as_index(), now).unwrap();
+    repo.store().gc(base_index.as_index(), now()).unwrap();
     assert_eq!(collect_no_gc_refs(&git_repo), hashset! {});
 }