Rename git_tree_entry_byoid() to _byid() as per the convention.

This wasn't being tested and since it has a callback, I fixed it
even though the return value of this callback is not treated like
any of the other callbacks in the API.

When a tool needs to recreate the tree object (for example an
interface to another VCS), it needs to use the raw attributes,
forgoing any normalization.

Fixed a few header @param and @return typos with the help of -Wdocumentation in Xcode.

The following warnings have not been fixed:
common.h:213 - Not sure how the documentation format is for '...'
notes.h:102 - Correct @param name but empty text
notes.h:111 - Correct @param name but empty text
pack.h:140 - @return missing text
pack.h:148 - @return missing text

This adds an example implementation that emulates git cat-file.
It is a convenient and relatively simple example of getting data
out of a repository.

Implementing this also revealed that there are a number of APIs
that are still not using const pointers to objects that really
ought to be.  The main cause of this is that `git_vector_bsearch`
may need to call `git_vector_sort` before doing the search, so a
const pointer to the vector is not allowed.  However, for tree
objects, with a little care, we can ensure that the vector of
tree entries is always sorted and allow lookups to take a const
pointer.  Also, the missing const in commit objects just looks
like an oversight.

This removes the GIT_INLINE versions of the simple git_object
accessors and standardizes them with a helper macro in src/object.h
to build the function bodies.

Conflicts:
	src/tree.c