Check for compiler flag support instead of checking for a platform

Reported by coverity.

Returning an error cleared the buf, but this operation does not free the
memory associated with it. Use git_buf_free() instead.

Add support for SSH keyboard-interactive authentication

Fix several ignore and attribute file behavior bugs

Only apply LEADING_DIR pattern munging to patterns in ignore and
attribute files, not to pathspecs used to select files to operate
on.  Also, allow internal macro definitions to be evaluated before
loading all external ones (important so that external ones can
make use of internal `binary` definition).

Ignore patterns that ended with a trailing '/*' were still needing
to match against another actual '/' character in the full path.
This is not the same behavior as core Git.

Instead, we strip a trailing '/*' off of any patterns that were
matching and just take it to imply the FNM_LEADING_DIR behavior.

There was a latent bug where files that use macro definitions
could be parsed before the macro definitions were loaded.  Because
of attribute file caching, preloading files that are going to be
used doesn't add a significant amount of overhead, so let's always
preload any files that could contain macros before we assemble the
actual vector of files to scan for attributes.

When traversing the directory structure, the iterator pushes and
pops ignore files using a vector.  Some directories don't have
ignore files, so it uses a path comparison to decide when it is
right to actually pop the last ignore file.  This was only
comparing directory suffixes, though, so a subdirectory with the
same name as a parent could result in the parent's .gitignore
being popped off the list ignores too early.  This changes the
logic to compare the entire relative path of the ignore file.

This simplifies platform/compiler dependent checks where we optionally
enable features or disable warnings.

Introduce core.safecrlf handling

This allows for keyboard-interactive based SSH authentication

cred: tighten username rules

The ssh-specific credentials allow the username to be missing. The idea
being that the ssh transport will then use the username provided in the
url, if it's available. There are two main issues with this.

The credential callback already knows what username was provided by the
url and needs to figure out whether it wants to ask the user for it or
it can reuse it, so passing NULL as the username means the credential
callback is suspicious.

The username provided in the url is not in fact used by the
transport. The only time it even considers it is for the user/pass
credential, which asserts the existence of a username in its
constructor. For the ssh-specific ones, it passes in the username stored
in the credential, which is NULL. The libssh2 macro we use runs strlen()
against this value (which is no different from what we would be doing
ourselves), so we then crash.

As the documentation doesn't suggest to leave out the username, assert
the need for a username in the code, which removes this buggy behavior
and removes implicit state.

git_cred_has_username() becomes a blacklist of credential types that do
not have a username. The only one at the moment is the 'default' one,
which is meant to call up some Microsoft magic.

Make index iterator thread safe

We treat this as a NUL-terminated string, so make sure that we add the
terminator.

The checks to see if files were out of date in the attibute cache
was wrong because the cache-breaker data wasn't getting stored
correctly.  Additionally, when the cache-breaker triggered, the
old file data was being leaked.

I don't love this approach, but achieving thread-safety for
attribute and ignore data while reloading files would require a
larger rewrite in order to avoid this.  If an attribute or ignore
file is out of date, this holds a lock on the file while we are
reloading the data so that another thread won't try to reload the
data at the same time.

In the threading tests, I was still seeing a race condition where
the same item could end up being inserted multiple times into the
index.  Preserving the sorted-ness of the index outside of the
`index_insert` call fixes the issue.

This is a big refactoring of the attribute file cache to be a bit
simpler which in turn makes it easier to enforce a lock around any
updates to the cache so that it can be used in a threaded env.
Tons of changes to the attributes and ignores code.

While I was looking at the conflict cleanup code, I looked over at
the tree cache code, since we clear the tree cache for each entry
that gets removed and there is some redundancy there.  I made some
small tweaks to avoid extra calls to strchr and strlen in a few
circumstances.

I introduced a leak into conflict cleanup by removing items from
inside the git_vector_remove_matching call.  This simplifies the
code to just use one common way for the two conflict cleanup APIs.

When an index has an active snapshot, removing an item can cause
an error (inserting into the deferred deletion vector), so I made
the git_index_conflict_cleanup API return an error code.  I felt
like this wasn't so bad since it is just like the other APIs.

I fixed up a couple of comments while I was changing the header.

The iterator pushes and pops ignores incrementally onto a list as
it traverses the directory structure so that it doesn't have to
constantly recheck which ignore files apply.  With the new ref
counting, it wasn't decrementing the refcount on the ignores that
it removed from the vector.

Some ignore files were not being freed from the cache.

Clear up some of the various "find" functions and the snapshot
API naming to be things I like more.

This makes the lock management on the index a little bit broader,
having a number of routines hold the lock across looking up the
item to be modified and actually making the modification.  Still
not true thread safety, but more pure index modifications are now
safe which allows the simple cases (such as starting up a diff
while index modifications are underway) safe enough to get the
snapshot without hitting allocation problems.

As part of this, I simplified the allocation of index entries to
use a flex array and just put the path at the end of the index
entry.  This makes every entry self-contained and makes it a
little easier to feel sure that pointers to strings aren't
being accidentally copied and freed while other references are
still being held.

This adds a basic test of doing simultaneous diffs on multiple
threads and adds basic locking for the attr file cache because
that was the immediate problem that arose from these tests.

This makes the index iterator honor the GIT_ITERATOR_IGNORE_CASE
and GIT_ITERATOR_DONT_IGNORE_CASE flags without modifying the
index data itself.  To take advantage of this, I had to export a
number of the internal index entry comparison functions.  I also
wrote some new tests to exercise the capability.

This surrounds any function that mutates the entries vector with
a mutex so it can be safely snapshotted.