revparse: Free left side of invalid range revspecs

This fixes a small memory leak in git_revparse where early returns on
errors from git_revparse_single cause a free() on the (reallocated) left
side of the revspec to be skipped.

That commit accidentally removed the initialization of the "start"
variable giving undefined results for the host extraction from the
url input.

Respect GIT_SSL_NO_VERIFY and http.sslVerify

remote: relax the url rules

Test SSH in travis

fileops: stat() before open()ing in git_futils_readbuffer_updated()

Do not use the "experimental" lookup mode

Accept any value for the remote's url, including an empty string which
we used to reject as invalid configuration.

This is not quite what git does (although it has its own problems with
such configurations) and it makes it harder to fix the issue, by not
letting the user modify it.

As we already need to check for a valid URL when we try to connect to
the network, let that perform the check, as we don't need to do it
anywhere else.

Set up the ssh credentials so we are able to talk to localhost and
issue git commands. Move to use a script, as the command list is
getting somewhat long.

While here, delay installing valgrind until we need it, as it and its
dependencies are by far the largest downloads and this allows us to
start compiling (and failing) faster and we only incur this cost when
the test suite runs successfully.

Speed up build under MSVC

Bigger buffer for utf-8 parsing in win32

This reverts refactoring done in 13224ea4
that introduces a performance regression for NFS when reading files that
don't exist. open() forces a cache invalidation on NFS, while stat()ing a
file just uses the cache and is very quick.

To give a specific example, say you have a repo with a thousand packed
refs. Before this change, looking up every single one ould incur a thousand
slow open() calls. With this change, it's a thousand fast stat() calls.

This is just a bunch of small fixes that I noticed while looking
at the UTF8 and UTF16 path stuff.  It fixes a slowdown in looking
for an empty directory (not exiting loop asap), makes the dir name
in the git__DIR structure be a GIT_FLEX_ARRAY to save an allocation,
and fixes some slightly odd assumptions in the cl_getenv helper.

SSH username fixes

With the current code, running 'cmake .' in an already-configured
directory causes the removal of ssh flags passed to the compiler,
making it impossible to build with ssh support but by removing CMake's
cache.

Remove the check for LIBSSH2_LIBRARY and let CMake do the right thing
wrt finding the library.

Key-based authentication also needs an username, so include it in each
one.

Also stop assuming a default username of "git" in the ssh transport
which has no business making such a decision.

By the time we recognise this as an ambiguous id, the object's data
has been loaded into memory. Free it when returning EABMIGUOUS.

Fix issue 1766 - bugs in managing ignore file lists

Fixes to safely reading the index

This just cleans up the improved logic for popping ignore dirs
and documents why the complex behavior is needed.