completion tests: consolidate getting path of current working directory

completion tests: consolidate getting path of current working directory

Some tests of the __gitdir() helper function use the $TRASH_DIRECTORY
variable in direct path comparisons.  In general this should be
avoided, because it might contain symbolic links.  There happens to be
no issues with this here, however, because those tests use
$TRASH_DIRECTORY both for specifying the expected result and for
specifying input which in turn is just 'echo'ed verbatim.

Other __gitdir() tests ask for the path of the trash directory by
running $(pwd -P) in each test, sometimes even twice in a single test.

Run $(pwd) only once at the beginning of the test script to store the
path of the trash directory in a variable, and use that variable in
all __gitdir() tests.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>