[API] Get a single commit via Ref (#10915)

* GET /repos/:owner/:repo/commits/:ref

* add Validation Checks

* Fix & Extend TEST

* add two new tast cases

modules/git/sha1.go

@@ -8,6 +8,7 @@ package git
 import (
 	"encoding/hex"
 	"fmt"
+	"regexp"
 	"strings"
 
 	"github.com/go-git/go-git/v5/plumbing"
@@ -16,6 +17,9 @@ import (
 // EmptySHA defines empty git SHA
 const EmptySHA = "0000000000000000000000000000000000000000"
 
+// SHAPattern can be used to determine if a string is an valid sha
+var SHAPattern = regexp.MustCompile(`^[0-9a-f]{4,40}$`)
+
 // SHA1 a git commit name
 type SHA1 = plumbing.Hash