Move git command to git/gitcmd (#35483)

The name cmd is already used in many places and may cause conflicts, so
I chose `gitcmd` instead to minimize potential naming conflicts.

modules/git/gitcmd/env.go

@@ -0,0 +1,40 @@
+// Copyright 2025 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package gitcmd
+
+import (
+	"fmt"
+	"os/exec"
+
+	"code.gitea.io/gitea/modules/log"
+	"code.gitea.io/gitea/modules/setting"
+)
+
+var GitExecutable = "git" // the command name of git, will be updated to an absolute path during initialization
+
+// SetExecutablePath changes the path of git executable and checks the file permission and version.
+func SetExecutablePath(path string) error {
+	// If path is empty, we use the default value of GitExecutable "git" to search for the location of git.
+	if path != "" {
+		GitExecutable = path
+	}
+	absPath, err := exec.LookPath(GitExecutable)
+	if err != nil {
+		return fmt.Errorf("git not found: %w", err)
+	}
+	GitExecutable = absPath
+	return nil
+}
+
+// HomeDir is the home dir for git to store the global config file used by Gitea internally
+func HomeDir() string {
+	if setting.Git.HomePath == "" {
+		// strict check, make sure the git module is initialized correctly.
+		// attention: when the git module is called in gitea sub-command (serv/hook), the log module might not obviously show messages to users/developers.
+		// for example: if there is gitea git hook code calling NewCommand before git.InitXxx, the integration test won't show the real failure reasons.
+		log.Fatal("Unable to init Git's HomeDir, incorrect initialization of the setting and git modules")
+		return ""
+	}
+	return setting.Git.HomePath
+}