filters: custom filters with wildcard attributes

Don't add unnecessary trailing newline during file merge

Upgrade xdiff to version used in core git 2.4.5 (0df0541).

Corrects an issue where an LF is added at EOF while applying
an unrelated change (ba31180), cleans up some unused code (be89977 and
e5b0662), and provides an improved callback to avoid leaking internal
(to xdiff) structures (467d348).

This also adds some additional functionality that we do not yet take
advantage of, namely the ability to ignore changes whose lines are
all blank (36617af).

When invoked with three files that each lack a trailing newline,
the merge result should also lack a trailing newline.

meta: Add Antoine Pelisse to hall-of-fame list

Antoine Pelisse <apelisse@gmail.com> has kindly allowed his
contributions to core git to be used under the libgit2 license.

Added git_diff_index_to_index()

git__getenv: utf-8 aware env reader

Stacktraces with CRTDBG memory leaks on Windows

Homebrew will error out because it's already installed.

We need to make sure we are asking for the current version of packages,
or we might get 404s from the download service.

filter: add docs for streaming filters

These functions are available on the public API but don't have any
documentation, so they don't appear on the API reference. Fix that.

iterator_walk: cast away constness for free

Introduce `git__getenv` which is a UTF-8 aware `getenv` everywhere.
Make `cl_getenv` use this to keep consistent memory handling around
return values (free everywhere, as opposed to only some platforms).

filter::stream: free the filter sanely

Don't use the filter's free callback to free the actual data structure
holding the filter, as we may not always actually initialize it (the
test may be skipped).

Correctly delimit the keys for submodule lookup

The regex we use to look at the gitmodules file does not correctly
delimit the name of submodule which we want to look up and puts '.*'
straight after the name, maching on any submodule which has the seeked
submodule as a prefix of its name.

Add the missing '\.' in the regex so we want a full stop to exist both
before and after the submodule name.

When two submodules are fairly similar, we may end up loading the wrong
one.

Fix 8.3 filename tests failure when 8.3 is disabled

The function was removed, but its declaration and changelog entry about
its removal were forgotten.

The comment in the test doesn't make any sense as the function doesn't
exist anymore, so get rid of it as well.

Allow custom filters with wildcard attributes, so that clients
can support some random `filter=foo` in a .gitattributes and look
up the corresponding smudge/clean commands in the configuration file.

Fix #3093 - remove declaration of unused function git_fetch__download…

Function was added in commit 2c982daa on October 5, 2011,
and removed in commit 41fb1ca0 on October 29, 2012.
Given the length of time it's gone unused, it's safe to remove now.

More warnings

http: fixed leak when asking for credentials again

t->cred might have been allocated the previous time and needs to be
freed before asking caller for credentials again.