Propagate failures caused by pool initialization errors.

Instead of using a signed type (`off_t`) use a new `git_object_size_t`
for the sizes of objects.

When `GIT_ATTR_CHECK_INCLUDE_HEAD` is specified, read `gitattribute`
files that are checked into the repository at the HEAD revision.

Right now, we are unconditionally applying all macros found in a
gitatttributes file. But quoting gitattributes(5):

    Custom macro attributes can be defined only in top-level
    gitattributes files ($GIT_DIR/info/attributes, the .gitattributes
    file at the top level of the working tree, or the global or
    system-wide gitattributes files), not in .gitattributes files in
    working tree subdirectories. The built-in macro attribute "binary"
    is equivalent to:

So gitattribute files in subdirectories of the working tree may
explicitly _not_ contain macro definitions, but we do not currently
enforce this limitation.

This patch introduces a new parameter to the gitattributes parser that
tells whether macros are allowed in the current file or not. If set to
`false`, we will still parse macros, but silently ignore them instead of
adding them to the list of defined macros. Update all callers to
correctly determine whether the to-be-parsed file may contain macros or
not. Most importantly, when walking up the directory hierarchy, we will
only set it to `true` once it reaches the root directory of the repo
itself.

Add a test that verifies that we are indeed not applying macros from
subdirectories. Previous to these changes, the test would've failed.

The gitattributes code is one of our oldest and most-untouched codebases
in libgit2, and as such its code style doesn't quite match our current
best practices. Refactor the function `git_attr_file__parse_buffer` to
better match them.

The gitattributes code is one of our oldest and most-untouched codebases
in libgit2, and as such its code style doesn't quite match our current
best practices. Refactor the function `git_attr_file__lookup_standalone`
to better match them.

The function `git_attr_session__init` is currently only initializing
setting up the attribute's session key by incrementing the repo-global
key by one. Most notably, all other members of the `git_attr_session`
struct are not getting initialized at all. So if one is to allocate a
session on the stack and then calls `git_attr_session__init`, the
session will still not be fully initialized. We have fared just fine
with that until now as all users of the function have allocated the
session structure as part of bigger structs with `calloc`, and thus its
contents have been zero-initialized implicitly already.

Fix this by explicitly zeroing out the session to enable allocation of
sessions on the stack.

Upstream git has converted to use `wildmatch` instead of
`fnmatch`. Convert our gitattributes logic to use `wildmatch` as
the last user of `fnmatch`. Please, don't expect I know what I'm
doing here: the fnmatch parser is one of the most fun things to
play around with as it has a sh*tload of weird cases. In all
honesty, I'm simply relying on our tests that are by now rather
comprehensive in that area.

The conversion actually fixes compatibility with how git.git
parser "**" patterns when the given path does not contain any
directory separators. Previously, a pattern "**.foo" erroneously
wouldn't match a file "x.foo", while git.git would match.

Remove the new-unused LEADINGDIR/NOLEADINGDIR flags for
`git_attr_fnmatch`.

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.

In our attributes pattern parsing code, we have a comment that
states we might have to convert '\' characters to '/' to have
proper POSIX paths. But in fact, '\' characters are valid inside
the string and act as escape mechanism for various characters,
which is why we never want to convert those to POSIX directory
separators. Furthermore, gitignore patterns are specified to only
treat '/' as directory separators.

Remove the comment to avoid future confusion.