Fix crlf handling, particularly when autocrlf=true

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.

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).

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).

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

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.

Nul terminated oid

Fix link issue in network examples

Remove GIT_SUCCESS from documentation

Update clar and some test helpers clean up

This adds a helper function for the cases where you want to
quickly set a single boolean config value for a repository.
This allowed me to remove a lot of code.

This makes the size_t comparison test nicer (assuming that the
values are actually not using the full length), and converts
some cases that were using it for pointer comparison to use the
macro that is designed for pointer comparison.

Explain it in local-upstream branch terms so it's easier to grasp than
with the `one` and `two` naming from the merge-base code.

Three submodule status bug fixes

1. Fix sort order problem with submodules where "mod" was sorting
   after "mod-plus" because they were being sorted as "mod/" and
   "mod-plus/".  This involved pushing the "contains a .git entry"
   test significantly lower in the stack.
2. Reinstate behavior that a directory which contains a .git entry
   will be treated as a submodule during iteration even if it is
   not yet added to the .gitmodules.
3. Now that any directory containing .git is reported as submodule,
   we have to be more careful checking for GIT_EEXISTS when we
   do a submodule lookup, because that is the error code that is
   returned by git_submodule_lookup when you try to look up a
   directory containing .git that has no record in gitmodules or
   the index.

Implement opts interface for global/system file search paths

Get rid of some dead code, tighten things up a bit, and fix a bug
with core::env test.

This switches the APIs for setting and getting the global/system
search paths from using git_strarray to using a simple string with
GIT_PATH_LIST_SEPARATOR delimited paths, just as the environment
PATH variable would contain.  This makes it simpler to get and set
the value.

I also added code to expand "$PATH" when setting a new value to
embed the old value of the path.  This means that I no longer
require separate actions to PREPEND to the value.

Several warnings detected by static code analyzer fixed

Failing config related test

MSVC cmake improvements

Implicit type conversion argument of function to size_t type
Suspicious sequence of types castings: size_t -> int -> size_t
Consider reviewing the expression of the 'A = B == C' kind. The expression is calculated as following: 'A = (B == C)'
Unsigned type is never < 0

(see http://public.kitware.com/Bug/view.php?id=11240)

This is useful when linking libgit2 statically, as the setting must match the linking program's one.

The goal of this work is to expose the search logic for "global",
"system", and "xdg" files through the git_libgit2_opts() interface.

Behind the scenes, I changed the logic for finding files to have a
notion of a git_strarray that represents a search path and to store
a separate search path for each of the three tiers of config file.
For each tier, I implemented a function to initialize it to default
values (generally based on environment variables), and then general
interfaces to get it, set it, reset it, and prepend new directories
to it.

Next, I exposed these interfaces through the git_libgit2_opts
interface, reusing the GIT_CONFIG_LEVEL_SYSTEM, etc., constants
for the user to control which search path they were modifying.
There are alternative designs for the opts interface / argument
ordering, so I'm putting this phase out for discussion.

Additionally, I ended up doing a little bit of clean up regarding
attr.h and attr_file.h, adding a new attrcache.h so the other two
files wouldn't have to be included in so many places.

Further tree_iterator refactoring

Fix various build warnings