Fixup git_attr_value's comment to be recognised as documentation, and
include the definitions needed for clang to parse reset.h such that
it shows up in the documentation.
This fixes#2430.
This expands the types of peeling that `git_object_peel` knows
how to do to include TAG -> BLOB peeling, and makes the errors
slightly more consistent depending on the situation. It also
adds a new special behavior where peeling to ANY will peel until
the object type changes (e.g. chases TAGs to a non-TAG).
Using this expanded peeling, this replaces peeling code that was
embedded in `git_tag_peel` and `git_reset`.
