Refactor Branch struct in package modules/git (#33980)

The `Branch` struct in `modules/git` package is unnecessary. We can just
use a `string` to represent a branch

models/fixtures/branch.yml

@@ -93,3 +93,111 @@
   is_deleted: false
   deleted_by_id: 0
   deleted_unix: 0
+
+-
+  id: 16
+  repo_id: 16
+  name: 'master'
+  commit_id: '69554a64c1e6030f051e5c3f94bfbd773cd6a324'
+  commit_message: 'not signed commit'
+  commit_time: 1502042309
+  pusher_id: 2
+  is_deleted: false
+  deleted_by_id: 0
+  deleted_unix: 0
+
+-
+  id: 17
+  repo_id: 16
+  name: 'not-signed'
+  commit_id: '69554a64c1e6030f051e5c3f94bfbd773cd6a324'
+  commit_message: 'not signed commit'
+  commit_time: 1502042309
+  pusher_id: 2
+  is_deleted: false
+  deleted_by_id: 0
+  deleted_unix: 0
+
+-
+  id: 18
+  repo_id: 16
+  name: 'good-sign-not-yet-validated'
+  commit_id: '27566bd5738fc8b4e3fef3c5e72cce608537bd95'
+  commit_message: 'good signed commit (with not yet validated email)'
+  commit_time: 1502042234
+  pusher_id: 2
+  is_deleted: false
+  deleted_by_id: 0
+  deleted_unix: 0
+
+-
+  id: 19
+  repo_id: 16
+  name: 'good-sign'
+  commit_id: 'f27c2b2b03dcab38beaf89b0ab4ff61f6de63441'
+  commit_message: 'good signed commit'
+  commit_time: 1502042101
+  pusher_id: 2
+  is_deleted: false
+  deleted_by_id: 0
+  deleted_unix: 0
+
+-
+  id: 20
+  repo_id: 1
+  name: 'feature/1'
+  commit_id: '65f1bf27bc3bf70f64657658635e66094edbcb4d'
+  commit_message: 'Initial commit'
+  commit_time: 1489950479
+  pusher_id: 2
+  is_deleted: false
+  deleted_by_id: 0
+  deleted_unix: 0
+
+-
+  id: 21
+  repo_id: 49
+  name: 'master'
+  commit_id: 'aacbdfe9e1c4b47f60abe81849045fa4e96f1d75'
+  commit_message: "Add 'test/test.txt'"
+  commit_time: 1572535577
+  pusher_id: 2
+  is_deleted: false
+  deleted_by_id: 0
+  deleted_unix: 0
+
+-
+  id: 22
+  repo_id: 1
+  name: 'develop'
+  commit_id: '65f1bf27bc3bf70f64657658635e66094edbcb4d'
+  commit_message: "Initial commit"
+  commit_time: 1489927679
+  pusher_id: 1
+  is_deleted: false
+  deleted_by_id: 0
+  deleted_unix: 0
+
+-
+  id: 23
+  repo_id: 3
+  name: 'master'
+  commit_id: '2a47ca4b614a9f5a43abbd5ad851a54a616ffee6'
+  commit_message: "init project"
+  commit_time: 1497448461
+  pusher_id: 1
+  is_deleted: false
+  deleted_by_id: 0
+  deleted_unix: 0
+
+-
+  id: 24
+  repo_id: 3
+  name: 'test_branch'
+  commit_id: 'd22b4d4daa5be07329fcef6ed458f00cf3392da0'
+  commit_message: "test commit"
+  commit_time: 1602935385
+  pusher_id: 1
+  is_deleted: false
+  deleted_by_id: 0
+  deleted_unix: 0