object-file: use real paths when adding alternates

object-file: use real paths when adding alternates

When adding an alternate ODB, we check if the alternate has the same
path as the object dir, and if so, we do nothing. However, that
comparison does not resolve symlinks. This makes it possible to add the
object dir as an alternate, which may result in bad behavior. For
example, it can trick "git repack -a -l -d" (possibly run by "git gc")
into thinking that all packs come from an alternate and delete all
objects.

	rm -rf test &&
	git clone https://github.com/git/git test &&
	(
	cd test &&
	ln -s objects .git/alt-objects &&
	# -c repack.updateserverinfo=false silences a warning about not
	# being able to update "info/refs", it isn't needed to show the
	# bad behavior
	GIT_ALTERNATE_OBJECT_DIRECTORIES=".git/alt-objects" git \
		-c repack.updateserverinfo=false repack -a -l -d  &&
	# It's broken!
	git status
	# Because there are no more objects!
	ls .git/objects/pack
	)

Fix this by resolving symlinks and relative paths before comparing the
alternate and object dir. This lets us clean up a number of issues noted
in 37a95862c6 (alternates: re-allow relative paths from environment,
2016-11-07):

- Now that we compare the real paths, duplicate detection is no longer
  foiled by relative paths.
- Using strbuf_realpath() allows us to "normalize" paths that
  strbuf_normalize_path() can't, so we can stop silently ignoring errors
  when "normalizing" paths from the environment.
- We now store an absolute path based on getcwd() (the "future
  direction" named in 37a95862c6), so chdir()-ing in the process no
  longer changes the directory pointed to by the alternate. This is a
  change in behavior, but a desirable one.

Signed-off-by: Glen Choo <chooglen@google.com>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>