When the user passes in a diff which has no repository associated
we may call `git_config__get_int_force` with a NULL-pointer
configuration. Even though `git_config__get_int_force` is
designed to swallow errors, it is not intended to be called with
a NULL pointer configuration.

Fix the issue by only calling `git_config__get_int_force` only
when configuration could be retrieved from the repository.

In C89 it is undefined behavior to pass `NULL` pointers to
`strncmp` and later on in C99 it has been explicitly stated that
functions with an argument declared as `size_t nmemb` specifying
the array length shall always have valid parameters, no matter if
`nmemb` is 0 or not (see ISO 9899 §7.21.1.2).

The function `str_equal_no_trailing_slash` always passes its
parameters to `strncmp` if their lengths match. This means if one
parameter is `NULL` and the other one either `NULL` or a string
with length 0 we will pass the pointers to `strncmp` and cause
undefined behavior.

Fix this by explicitly handling the case when both lengths are 0.

When computing a short OID we do this by first copying the
leading parts into the new OID structure and then setting the
trailing part to zero. In the case of the desired length being
`GIT_OID_HEXSZ - 1` we will call `memset` with an out of bounds
pointer and a length of 0. While this seems to cause no problems
for common platforms the C89 standard does not explicitly state
that calling `memset` with an out of bounds pointer and
length of 0 is valid.

Fix the potential issue by using the newly introduced
`git_oid__cpy_prefix` function.

When parsing a section header we expect something along the
format of '[section "subsection"]'. When a section is
mal-formated and is entirely missing its quotation marks we catch
this case by observing that `strchr(line, '"') - strrchr(line,
'"') = NULL - NULL = 0` and error out. Unfortunately, the error
message is misleading though, as we state that we are missing the
closing quotation mark while we in fact miss both quotation
marks.

Improve the error message by explicitly checking if the first
quotation mark could be found and, if not, stating that quotation
marks are completely missing.

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.