chore: Refactor Is{Reference,Branch}Exist (#8425)

- Instead of invoking a new git command to check if a reference exists, use the already opened git repository that uses a batch check to check if the reference exists.
- Instead of invoking a new git command to check if a branch exists, use the already opened git repository that uses a batch check to check if the branch exists.
- Do not depend on `IsReferenceExist` as its no longer used by any other function.
- Specify `--quiet` to not waste Git's time on printing something we do not use.
- Run it via `Run` so no buffers are created for stdout and stderr.
- Add a test that verifies `IsBranchExist` works and does not work for other types of references.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/8425
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: Gusted <postmaster@gusted.xyz>
Co-committed-by: Gusted <postmaster@gusted.xyz>

modules/git/repo_branch_test.go

@@ -1,4 +1,5 @@
 // Copyright 2018 The Gitea Authors. All rights reserved.
+// Copyright 2025 The Forgejo Authors. All rights reserved.
 // SPDX-License-Identifier: MIT
 
 package git
@@ -195,3 +196,17 @@ func TestRepository_IsReferenceExist(t *testing.T) {
 		})
 	}
 }
+
+func TestIsBranchExist(t *testing.T) {
+	repo1Path := filepath.Join(testReposDir, "repo1_bare")
+
+	assert.True(t, IsBranchExist(t.Context(), repo1Path, "branch1"))
+	assert.True(t, IsBranchExist(t.Context(), repo1Path, "branch2"))
+	assert.True(t, IsBranchExist(t.Context(), repo1Path, "master"))
+
+	assert.False(t, IsBranchExist(t.Context(), repo1Path, "HEAD"))
+	assert.False(t, IsBranchExist(t.Context(), repo1Path, "153f451b9ee7fa1da317ab17a127e9fd9d384310"))
+	assert.False(t, IsBranchExist(t.Context(), repo1Path, "153f451b9ee7fa1da317ab17a127e9fd9d384310"))
+	assert.False(t, IsBranchExist(t.Context(), repo1Path, "signed-tag"))
+	assert.False(t, IsBranchExist(t.Context(), repo1Path, "test"))
+}