Avoid chdir() in list_commands_in_dir()

Avoid chdir() in list_commands_in_dir()

The function list_commands_in_dir() tried to be lazy and just chdir()
to the directory which entries it listed, so that the check if the
file is executable could be done on dir->d_name.

However, there is no good reason to jump around wildly just to find
all Git commands.

Instead, have a strbuf and construct the full path dynamically.

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