Ensure memcache TTL cannot be over 30 days (#14592)

Memcached TTL cannot be > 30 days and if it is attempted the TTL is interpreted as
a unix timestamp.

This PR ensures that the TTL is switched to a unix timestamp in those cases.

Fix #14571

Signed-off-by: Andrew Thornton <art27@cantab.net>

modules/git/last_commit_cache_nogogit.go

@@ -13,14 +13,14 @@ import (
 // LastCommitCache represents a cache to store last commit
 type LastCommitCache struct {
 	repoPath    string
-	ttl         int64
+	ttl         func() int64
 	repo        *Repository
 	commitCache map[string]*Commit
 	cache       Cache
 }
 
 // NewLastCommitCache creates a new last commit cache for repo
-func NewLastCommitCache(repoPath string, gitRepo *Repository, ttl int64, cache Cache) *LastCommitCache {
+func NewLastCommitCache(repoPath string, gitRepo *Repository, ttl func() int64, cache Cache) *LastCommitCache {
 	if cache == nil {
 		return nil
 	}