run-command API: make "n" parameter a "size_t"

Make the "n" variable added in c553c72eed6 (run-command: add an
asynchronous parallel child processor, 2015-12-15) a "size_t". As
we'll see in a subsequent commit we do pass "0" here, but never "jobs
< 0".

We could have made it an "unsigned int", but as we're having to change
this let's not leave another case in the codebase where a size_t and
"unsigned int" size differ on some platforms. In this case it's likely
to never matter, but it's easier to not need to worry about it.

After this and preceding changes:

	make run-command.o DEVOPTS=extra-all CFLAGS=-Wno-unused-parameter

Only has one (and new) -Wsigned-compare warning relevant to a
comparison about our "n" or "{nr,max}_processes": About using our
"n" (size_t) in the same expression as online_cpus() (int). A
subsequent commit will adjust & deal with online_cpus() and that
warning.

The only users of the "n" parameter are:

 * builtin/fetch.c: defaults to 1, reads from the "fetch.parallel"
   config. As seen in the code that parses the config added in
   d54dea77dba (fetch: let --jobs=<n> parallelize --multiple, too,
   2019-10-05) will die if the git_config_int() return value is < 0.

   It will however pass us n = 0, as we'll see in a subsequent commit.

 * submodule.c: defaults to 1, reads from "submodule.fetchJobs"
   config. Read via code originally added in a028a1930c6 (fetching
   submodules: respect `submodule.fetchJobs` config option, 2016-02-29).

   It now piggy-backs on the the submodule.fetchJobs code and
   validation added in f20e7c1ea24 (submodule: remove
   submodule.fetchjobs from submodule-config parsing, 2017-08-02).

   Like builtin/fetch.c it will die if the git_config_int() return
   value is < 0, but like builtin/fetch.c it will pass us n = 0.

 * builtin/submodule--helper.c: defaults to 1. Read via code
   originally added in 2335b870fa7 (submodule update: expose parallelism
   to the user, 2016-02-29).

   Since f20e7c1ea24 (submodule: remove submodule.fetchjobs from
   submodule-config parsing, 2017-08-02) it shares a config parser and
   semantics with the submodule.c caller.

 * hook.c: hardcoded to 1, see 96e7225b310 (hook: add 'run'
   subcommand, 2021-12-22).

 * t/helper/test-run-command.c: can be -1 after parsing the arguments,
   but will then be overridden to online_cpus() before passing it to
   this API. See be5d88e1128 (test-tool run-command: learn to run (parts
   of) the testsuite, 2019-10-04).

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

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