libgit2 has two distinct requirements that were previously solved by
`git_buf`.  We require:

1. A general purpose string class that provides a number of utility APIs
   for manipulating data (eg, concatenating, truncating, etc).
2. A structure that we can use to return strings to callers that they
   can take ownership of.

By using a single class (`git_buf`) for both of these purposes, we have
confused the API to the point that refactorings are difficult and
reasoning about correctness is also difficult.

Move the utility class `git_buf` to be called `git_str`: this represents
its general purpose, as an internal string buffer class.  The name also
is an homage to Junio Hamano ("gitstr").

The public API remains `git_buf`, and has a much smaller footprint.  It
is generally only used as an "out" param with strict requirements that
follow the documentation.  (Exceptions exist for some legacy APIs to
avoid breaking callers unnecessarily.)

Utility functions exist to convert a user-specified `git_buf` to a
`git_str` so that we can call internal functions, then converting it
back again.

Upstream git.git has converted its codebase to use wildcard in
favor of fnmatch in commit 70a8fc999d (stop using fnmatch (either
native or compat), 2014-02-15). To keep our own regex-matching in
line with what git does, convert all trivial instances of
`fnmatch` usage to use `wildcard`, instead. Trivial usage is
defined to be use of `fnmatch` with either no flags or flags that
have a 1:1 equivalent in wildmatch (PATHNAME, IGNORECASE).

We're about to phase out our bundled fnmatch implementation as
git.git has moved to wildmatch long ago in 2014. To make it
easier to spot which files are stilll using fnmatch, remove the
implicit "fnmatch.h" include in "posix.h" and instead include it
explicitly.

Assorted Coverity fixes

C++ style comment ("//") are not specified by the ISO C90 standard and
thus do not conform to it. While libgit2 aims to conform to C90, we did
not enforce it until now, which is why quite a lot of these
non-conforming comments have snuck into our codebase. Do a tree-wide
conversion of all C++ style comments to the supported C style comments
to allow us enforcing strict C90 compliance in a later commit.

The path given to `git_index_add_bypath` is relative to the root of the
repository. That `describe/file` path is relative to the root of the
sandbox directory, hence if I add the missing `cl_git_pass` I rightfully
get an error that `$SANDBOX/describe/describe/file doesn't exist`.

The path is thus changed to be made relative to the repository, which
makes the failure go away and "restore" the test.

Reported by Coverity, CID 1393678-1393697.

When no reference names could be found we did error out when trying to describe
a commit. This is wrong, though, when the option to fall back to a commit's
object ID is set.

The signature for the reflog is not something which changes
dynamically. Almost all uses will be NULL, since we want for the
repository's default identity to be used, making it noise.

In order to allow for changing the identity, we instead provide
git_repository_set_ident() and git_repository_ident() which allow a user
to override the choice of signature.