Deprecate `git_filter_list_apply_to_data` as it takes user input as a
`git_buf`.  Users should use `git_filter_list_apply_to_buffer` instead.

`git_filter_list_stream_data` takes user input in a `git_buf`.
`git_buf` should only be used when libgit2 itself needs to allocate data
and returned to a user that they can free when they wish.  Replace it
with `git_filter_list_stream_buffer` that takes a data buffer and
length.

The function `git_treebuilder_write_with_buffer` is unnecessary; it
is used internally as part of treebuilder writing, but it has little
use to external callers.  For callers that repeatedly write a
treebuilder, we can supply them with a buffer in the treebuilder struct
instead of recreating it.  For ourselves, when we want a single buffer
in our write loop, we can use an internal function.

The information about the type of a revision spec is not information
about the parser.  Name it accordingly, so that `git_revparse_mode_t`
is now `git_revspec_t`.  Deprecate the old name.

`GIT_BLOB_FILTER_ATTTRIBUTES_FROM_HEAD` is misspelled, it should be
`GIT_BLOB_FILTER_ATTRIBUTES_FROM_HEAD`, and it would be if it were not
for the MacBook Pro keyboard and my inattentiveness.

occured -> occurred

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

We should not be in the business of copying strings around for users.
We either return a strarray that can be freed, or we take one (and do
not mutate it).