t: run tests from a normalized working directory

t: run tests from a normalized working directory

Some tests make git perform actions that produce observable pathnames,
and have expectations on those paths. Tests run with $HOME set to a
$TRASH_DIRECTORY, and with their working directory the same
$TRASH_DIRECTORY, although these paths are logically identical, they do
not observe the same pathname canonicalization rules and thus might not
be represented by strings that compare equal. In particular, no pathname
normalization is applied to $TRASH_DIRECTORY or $HOME, while tests
change their working directory with `cd -P`, which normalizes the
working directory's path by fully resolving symbolic links.

t7900's macOS maintenance tests (which are not limited to running on
macOS) have an expectation on a path that `git maintenance` forms by
using abspath.c strbuf_realpath() to resolve a canonical absolute path
based on $HOME. When t7900 runs from a working directory that contains
symbolic links in its pathname, $HOME will also contain symbolic links,
which `git maintenance` resolves but the test's expectation does not,
causing a test failure.

Align $TRASH_DIRECTORY and $HOME with the normalized path as used for
the working directory by resetting them to match the working directory
after it's established by `cd -P`. With all paths in agreement and
symbolic links resolved, pathname expectations can be set and met based
on string comparison without regard to external environmental factors
such as the presence of symbolic links in a path.

Suggested-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Mark Mentovai <mark@chromium.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>