help: do not expect built-in commands to be hardlinked

help: do not expect built-in commands to be hardlinked

When building with SKIP_DASHED_BUILT_INS=YesPlease, the built-in
commands are no longer present in the `PATH` as hardlinks to `git`.

As a consequence, `load_command_list()` needs to be taught to find the
names of the built-in commands from elsewhere.

This only affected the output of `git --list-cmds=main`, but not the
output of `git help -a` because the latter includes the built-in
commands by virtue of them being listed in command-list.txt.

The bug was detected via a patch series that turns the merge strategies
included in Git into built-in commands: `git merge -s help` relies on
`load_command_list()` to determine the list of available merge
strategies.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>