Make giterr_set_str() and giterr_set_oom() public APIs

Tags: teach git_tag_list not to include the 'refs/tags/' prefix

Topic/is valid name

Adjusts refs::list test (including the comments)
Adjusts objects::tags::list test

This patch changes the tag listing test helper to use a struct as input
parameter, which tells what we exactly expect.

As I don't think, we can rely on the fact that every os and every
filesystem will report the tags in the same order, I made this code
independent of the order that the tags are retrieved.

Since quite a while now, git_branch_foreach has learnt to list branches
without the 'refs/heads/' or 'refs/remotes' prefixes.
This patch teaches git_tag_list to do the same for listing tags.

Make clear that git_odb_hashfile does not use filters

Signed-off-by: Sven Strickroth <email@cs-ware.de>

There has been discussion for a while about making some set of
the `giterr_set` type functions part of the public API for code
that is implementing new backends to libgit2.  This makes the
`giterr_set_str()` and `giterr_set_oom()` functions public.

checkout: add notification callback for skipped files

git_repository_hashfile: Only close file handle if we have a valid one

Otherwise this throws an exception on MFC based systems.

Signed-off-by: Sven Strickroth <email@cs-ware.de>

Checkout, reset and others enhancements

Allocate a buffer large enough to store the path plus the terminator
instead of letting readlink write beyond the end.