run-command API: have run_process_parallel() take an "opts" struct

As noted in fd3aaf53f7 (run-command: add an "ungroup" option to
run_process_parallel(), 2022-06-07) which added the "ungroup" passing
it to "run_process_parallel()" via the global
"run_processes_parallel_ungroup" variable was a compromise to get the
smallest possible regression fix for "maint" at the time.

This follow-up to that is a start at passing that parameter and others
via a new "struct run_process_parallel_opts", as the earlier
version[1] of what became fd3aaf53f7 did.

Since we need to change all of the occurrences of "n" to
"opt->SOMETHING" let's take the opportunity and rename the terse "n"
to "processes". We could also have picked "max_processes", "jobs",
"threads" etc., but as the API is named "run_processes_parallel()"
let's go with "processes".

Since the new "run_processes_parallel()" function is able to take an
optional "tr2_category" and "tr2_label" via the struct we can at this
point migrate all of the users of "run_processes_parallel_tr2()" over
to it.

But let's not migrate all the API users yet, only the two users that
passed the "ungroup" parameter via the
"run_processes_parallel_ungroup" global

1. https://lore.kernel.org/git/cover-v2-0.8-00000000000-20220518T195858Z-avarab@gmail.com/

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

run-command.h

@@ -459,17 +459,64 @@ typedef int (*task_finished_fn)(int result,
 				void *pp_task_cb);
 
 /**
- * Runs up to n processes at the same time. Whenever a process can be
- * started, the callback get_next_task_fn is called to obtain the data
+ * Option used by run_processes_parallel(), { 0 }-initialized means no
+ * options.
+ */
+struct run_process_parallel_opts
+{
+	/**
+	 * tr2_category & tr2_label: sets the trace2 category and label for
+	 * logging. These must either be unset, or both of them must be set.
+	 */
+	const char *tr2_category;
+	const char *tr2_label;
+
+	/**
+	 * processes: see 'processes' in run_processes_parallel() below.
+	 */
+	size_t processes;
+
+	/**
+	 * ungroup: see 'ungroup' in run_processes_parallel() below.
+	 */
+	unsigned int ungroup:1;
+
+	/**
+	 * get_next_task: See get_next_task_fn() above. This must be
+	 * specified.
+	 */
+	get_next_task_fn get_next_task;
+
+	/**
+	 * start_failure: See start_failure_fn() above. This can be
+	 * NULL to omit any special handling.
+	 */
+	start_failure_fn start_failure;
+
+	/**
+	 * task_finished: See task_finished_fn() above. This can be
+	 * NULL to omit any special handling.
+	 */
+	task_finished_fn task_finished;
+
+	/**
+	 * data: user data, will be passed as "pp_cb" to the callback
+	 * parameters.
+	 */
+	void *data;
+};
+
+/**
+ * Options are passed via the "struct run_process_parallel_opts" above.
+ *
+ * Runs N 'processes' at the same time. Whenever a process can be
+ * started, the callback opts.get_next_task is called to obtain the data
  * required to start another child process.
  *
  * The children started via this function run in parallel. Their output
  * (both stdout and stderr) is routed to stderr in a manner that output
  * from different tasks does not interleave (but see "ungroup" below).
  *
- * start_failure_fn and task_finished_fn can be NULL to omit any
- * special handling.
- *
  * If the "ungroup" option isn't specified, the API will set the
  * "stdout_to_stderr" parameter in "struct child_process" and provide
  * the callbacks with a "struct strbuf *out" parameter to write output
@@ -479,19 +526,10 @@ typedef int (*task_finished_fn)(int result,
  * NULL "struct strbuf *out" parameter, and are responsible for
  * emitting their own output, including dealing with any race
  * conditions due to writing in parallel to stdout and stderr.
- * The "ungroup" option can be enabled by setting the global
- * "run_processes_parallel_ungroup" to "1" before invoking
- * run_processes_parallel(), it will be set back to "0" as soon as the
- * API reads that setting.
  */
-extern int run_processes_parallel_ungroup;
-void run_processes_parallel(size_t n,
-			    get_next_task_fn,
-			    start_failure_fn,
-			    task_finished_fn,
-			    void *pp_cb);
-void run_processes_parallel_tr2(size_t n, get_next_task_fn, start_failure_fn,
-				task_finished_fn, void *pp_cb,
+void run_processes_parallel(const struct run_process_parallel_opts *opts);
+void run_processes_parallel_tr2(size_t processes, get_next_task_fn,
+				start_failure_fn, task_finished_fn, void *pp_cb,
 				const char *tr2_category, const char *tr2_label);
 
 /**