git_buf_clear does not free allocated memory associated with a
git_buf. Use `git_buf_free` instead to correctly free its memory
and plug the memory leak.

rebase: additional setup tests of exotic behavior

appveyor: stop if the first test fails

Fixes for `gid_odb_expand_ids`

mwindow: free unused windows if we fail to mmap

The first time may be due to memory fragmentation or just bad luck on a
32-bit system. When we hit the mmap error for the first time, free up
the unused windows and try again.

The old implementation had two issues:

1. OIDs that were too short as to be ambiguous were not being handled
properly.

2. If the last OID to expand in the array was missing from the ODB, we
would leak a `GIT_ENOTFOUND` error code from the function.

Introduce `git_odb_expand_ids`

Take (and write to) an array of a struct, `git_odb_expand_id`.

Support for ssh+git and git+ssh protocols

commit: split creating the commit and writing it out

Sometimes you want to create a commit but not write it out to the
objectdb immediately. For these cases, provide a new function to
retrieve the buffer instead of having to go through the db.

Enable nanosecond resolution by default

Instead of hoping that we can get a racy entry by going real fast
and praying real hard, just create a racy entry.

Update link to Pro Git's Git internals chapter

If the underlying filesystem doesn't support better than one
second resolution, then don't expect that turning on `GIT_USE_NSEC`
does anything magical to change that.

Query the object database for multiple objects at a time, given their
object ID (which may be abbreviated) and optional type.

When looking up an abbreviated oid, show the actual (abbreviated) oid
the caller passed instead of a full (but ambiguously truncated) oid.

Update unit test to use newfangled `st_ctime_nsec`, which provides
indirection to the platform-correct name.

treebuilder: don't try to verify submodules exist in the odb

Submodules don't exist in the objectdb and the code is making us try to
look for a blob with its commit id, which is obviously not going to
work.

Skip the test if the user wants to insert a submodule.

ssh: initialize libssh2

We should have been doing this, but it initializes itself upon first
use, which works as long as nobody's doing concurrent network
operations. Initialize it on our init to make sure it's not getting
initialized concurrently.

Expand OpenSSL and libssh2 thread safety documentation

Nanosecond resolution is now the default in git itself.  Enable this
as our default as well.

test: make sure we retry the auth callback on all platforms

xdiff: fix memleak on error case

If the caller has provided bad authentication, give them another
apportunity to get it right until they give up. This brings WinHTTP in
line with the other transports.