Be more careful with user-supplied buffers

This adds in missing calls to `git_buf_sanitize` and fixes a
number of places where `git_buf` APIs could inadvertently write
NUL terminator bytes into invalid buffers.  This also changes the
behavior of `git_buf_sanitize` to NUL terminate a buffer if it can
and of `git_buf_shorten` to do nothing if it can.

Adds tests of filtering code with zeroed (i.e. unsanitized) buffer
which was previously triggering a segfault.
Pass unconverted data when iconv doesn't like it

When using Iconv to convert unicode data and iconv doesn't like
the source data (because it thinks that it's not actual UTF-8),
instead of stopping the operation, just use the unconverted data.
This will generally do the right thing on the filesystem, since
that is the source of the non-UTF-8 path data anyhow.

This adds some tests for creating and looking up branches with
messy Unicode names.  Also, this takes the helper function that
was previously internal to `git_repository_init` and makes it
into `git_path_does_fs_decompose_unicode` which is a useful in
tests to understand what the expected results should be.
Allow cl_repo_get_bool to work with missing key

One of the test helpers provides a quick way for looking up a
boolean key.  But if the key way missing completely, the check
would actually raise an error.  Given the way we use this helper,
if the key is missing, this should just return false, I think.
Don't always test composed-insensitive lookups

Only on a filesystem that is composed/decomposed insensitive,
should be testing that a branch can be looked up by the opposite
form and still work correctly.
Minor fixes for warnings and error propagation
Don't scale diff stat when not needed
Merge pull request #2334 from libgit2/rb/fix-2333

Be more careful with user-supplied buffers
Merge pull request #2336 from libgit2/rb/unicode-branch-names

Pass unconverted Unicode path data when iconv doesn't like it