This changes the get_entry() method to return a refcounted version of
the config entry, which you have to free when you're done.

This allows us to avoid freeing the memory in which the entry is stored
on a refresh, which may happen at any time for a live config.

For this reason, get_string() has been forbidden on live configs and a
new function get_string_buf() has been added, which stores the string in
a git_buf which the user then owns.

The functions which parse the string value takea advantage of the
borrowing to parse safely and then release the entry.

For consistency with the rest of the library, where an opt is an
options *structure*.

Add structures and preliminary functions to take a buffer, file or
blob and write the contents in chunks through an arbitrary number
of chained filters, finally writing into a user-provided function
accept the contents.

Fixed typo in git_repository_reinit_filesystem() documentation

It was missing "common.h" and "types.h" like other system headers.
This generated compilation errors if including it directly.

The implementation of the hashsig API disallows computing a signature on
small files containing only a few lines. This new flag disables this
behavior.

git_diff_find_similar() sets this flag by default which means that rename
/ copy detection of small files will now work. This in turn affects the
behavior of the git_status and git_blame APIs which will now detect rename
of small files assuming the right options are passed.