Incremental improvements to pack-objects logic

Incorporate feedback for incr. improvements to pack-objects

Support pthread_cond_* on Win32

Not found means not found, and the other way around.

Config location fixes

Fix single file ignores

test: fix some memory leaks

clone: Explicit support of no-checkout option

To answer if a single given file should be ignored, the path to
that file has to be processed progressively checking that there
are no intermediate ignored directories in getting to the file
in question.  This enables that, fixing the broken old behavior,
and adds tests to exercise various ignore situations.

Because fnmatch uses recursion, there were some input sequences
that cause seriously degenerate behavior.  This imports a fix
that imposes a max recursion limiter to avoid the worst of it.

reset: prevent hard reset in a bare repository

Test category for live network tests

Split the clone tests into network and no-network suites.

Also introduce the slective ref trimming promised but also missed in
the previous commit.

We used to require loose references to contain only an OID (possibly
after trimming the string). This is however not enough for letting us
lookup FETCH_HEAD, which can have a lot of content after the initial
OID.

Change the parsing rules so that a loose refernce must e at least 40
bytes long and the 41st (if it's there) must be accepted by
isspace(3). This makes the trim unnecessary, so only do it for
symrefs. This fixes #977.

 Enforce returning of EEXISTS when trying to overwrite a reference

Add link to D libgit2 bindings.

Also tell ctest and valgrind to run libgit2_clar with '-iall' so we
run the network tests in travis.

Add git_buf_put_base64 to buffer API

The fix for fetching from empty repositories (22935b06 protocol:
don't store flushes; 2012-10-07) forgot to take into account the
deletion of the flush pkt in the HTTP transport. As a result, the HEAD
ref advertisement where we detect the remote's capabilities was
deleted instead. Fix this.

[GSoC] RFC: pack objects