Add has_cr_in_index check to CRLF filter

This adds a check to the drop_crlf filter path to check it the
file in the index already has a CR in it, in which case this will
not drop the CRs from the workdir file contents.

This uncovered a "bug" in `git_blob_create_fromworkdir` where the
full path to the file was passed to look up the attributes instead
of the relative path from the working directory root.  This meant
that the check in the index for a pre-existing entry of the same
name was failing.
Added some tests for issue #1397

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Test fixes and cleanup

This fixes some places where the new tests were leaving the test
area in a bad state or were freeing data they should not free.
It also removes code that is extraneous to the core issue and
fixes an invalid SHA being looked up in one of the tests (which
was failing, but for the wrong reason).
don't convert CRLF to CRCRLF
Fix up checkout file contents checks

This fixes of the file contents checks in checkout to give
slightly better error messages by directly calling the underlying
clar assertions so the file and line number of the top level call
can be reported correctly, and renames the helpers to not start
with "test_" since that is kind of reserved by clar.

This also enables some of the CRLF tests on all platforms that
were previously Windows only (by pushing a check of the native
line endings into the test body).
Move crlf conversion into buf_text

This adds crlf/lf conversion functions into buf_text with more
efficient implementations that bypass the high level buffer
functions.  They attempt to minimize the number of reallocations
done and they directly write the buffer data as needed if they
know that there is enough memory allocated to memcpy data.

Tests are added for these new functions.  The crlf.c code is
updated to use the new functions.

Removed the include of buf_text.h from filter.h and just include
it more narrowly in the places that need it.
Merge pull request #1431 from libgit2/autocrlf-fixes

Fix crlf handling, particularly when autocrlf=true