run-command: extract sanitize_repo_env helper

The current prepare_other_repo_env() does two distinct things:

 1. Strip certain known environment variables that should be set by a
    child process based on a different repository.

 2. Set the GIT_DIR variable to avoid repository discovery.

The second item is valuable for child processes that operate on
submodules, where the repo discovery could be mistaken for the parent
repository.

In the next change, we will see an important case where only the first
item is required as the GIT_DIR discovery should happen naturally from
the '-C' parameter in the child process.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

run-command.c

@@ -1847,7 +1847,7 @@ int run_auto_maintenance(int quiet)
 	return run_command(&maint);
 }
 
-void prepare_other_repo_env(struct strvec *env, const char *new_git_dir)
+void sanitize_repo_env(struct strvec *env)
 {
 	const char * const *var;
 
@@ -1856,6 +1856,11 @@ void prepare_other_repo_env(struct strvec *env, const char *new_git_dir)
 		    strcmp(*var, CONFIG_COUNT_ENVIRONMENT))
 			strvec_push(env, *var);
 	}
+}
+
+void prepare_other_repo_env(struct strvec *env, const char *new_git_dir)
+{
+	sanitize_repo_env(env);
 	strvec_pushf(env, "%s=%s", GIT_DIR_ENVIRONMENT, new_git_dir);
 }
 

run-command.h

@@ -509,13 +509,18 @@ struct run_process_parallel_opts
  */
 void run_processes_parallel(const struct run_process_parallel_opts *opts);
 
+/**
+ * Unset all local-repo GIT_* variables in env; see local_repo_env in
+ * environment.h. GIT_CONFIG_PARAMETERS and GIT_CONFIG_COUNT are preserved
+ * to pass -c and --config-env options from the parent process.
+ */
+void sanitize_repo_env(struct strvec *env);
+
 /**
  * Convenience function which prepares env for a command to be run in a
- * new repo. This adds all GIT_* environment variables to env with the
- * exception of GIT_CONFIG_PARAMETERS and GIT_CONFIG_COUNT (which cause the
- * corresponding environment variables to be unset in the subprocess) and adds
- * an environment variable pointing to new_git_dir. See local_repo_env in
- * environment.h for more information.
+ * new repo. This removes variables pointing to the local repository (using
+ * sanitize_repo_env() above), and adds an environment variable pointing to
+ * new_git_dir.
  */
 void prepare_other_repo_env(struct strvec *env, const char *new_git_dir);