The `git_buf_text` namespace is unnecessary and strange.  Remove it,
just keep the functions prefixed with `git_buf`.

Move to the `git_error` name in the internal API for error-related
functions.

Examine the recent CRLF changes to git by Torsten Bögershausen and
include similar changes to update our CRLF logic to match.

Note: Torsten Bögershausen has previously agreed to allow his changes to
be included in libgit2.

The function to detect a BOM takes an offset where it shall look for a
BOM. No caller uses that, and searching for the BOM in the middle of a
buffer seems to be very unlikely, as a BOM should only ever exist at
file start.

Remove the parameter, as it has already caused confusion due to its
weirdness.

Allow files to have mixed line endings instead of skipping processing
on them.

Make our overflow checking look more like gcc and clang's, so that
we can substitute it out with the compiler instrinsics on platforms
that support it.  This means dropping the ability to pass `NULL` as
an out parameter.

As a result, the macros also get updated to reflect this as well.

Introduce `git_buf_grow_by` to incrementally increase the size of a
`git_buf`, performing an overflow calculation on the growth.

Introduce some helper macros to test integer overflow from arithmetic
and set error message appropriately.

Main change: Don't treat chars > 128 as non-printable (common in UTF-8 files)

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