sha1_file: always allow relative paths to alternates

We recursively expand alternates repositories, so that if A borrows from B which borrows from C, A can see all objects. For the root object database, we allow relative paths, so A can point to B as "../B/objects". However, we currently do not allow relative paths when recursing, so B must use an absolute path to reach C. That is an ancient protection from c2f493a (Transitively read alternatives, 2006-05-07) that tries to avoid adding the same alternate through two different paths. Since 5bdf0a8 (sha1_file: normalize alt_odb path before comparing and storing, 2011-09-07), we use a normalized absolute path for each alt_odb entry. This means that in most cases the protection is no longer necessary; we will detect the duplicate no matter how we got there (but see below). And it's a good idea to get rid of it, as it creates an unnecessary complication when setting up recursive alternates (B has to know that A is going to borrow from it and make sure to use an absolute path). Note that our normalization doesn't actually look at the filesystem, so it can still be fooled by crossing symbolic links. But that's also true of absolute paths, so it's not a good reason to disallow only relative paths (it's potentially a reason to switch to real_path(), but that's a separate and non-trivial change). We adjust the test script here to demonstrate that this now works, and add new tests to show that the normalization does indeed suppress duplicates. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>