pathspec: do not try to dereference NULL

pathspec_match_free() should not dereference a NULL passed to it.

I found this issue when I tried to run example log program with
nonexistent branch:

./example/log help

Such call leads to segmentation fault.

checkout: free last created directory

Remove logically dead code (we're already asserting)

README: Point to libgit2 account's AppVeyor badge

On case insensitive filesystems, we may have files in the working
directory that case fold to a name we want to write.  Remove those
files (by default) so that we will not end up with a filename that
has the unexpected case.

Symbolic links that abuse case insensitivity to write into .git.

Don't bother trying to recreate the previously created directory
during checkout, for a modest reduction in the number of stats.

Walk up the tree to mkdir, which is less immediately efficient,
but allows us to look at intermediate directories that may need
attention.

The documentation for `git_path_join_unrooted` states that the base
length will be returned, so that consumers like checkout know where
to start creating directories instead of always creating directories
at the directory root.

Checkout can now provide performance data about the number of (some)
syscalls performed using an optional callback.

Main change: Don't treat chars > 128 as non-printable (common in UTF-8 files)

Signed-off-by: Sven Strickroth <email@cs-ware.de>

diff: only compute patches when requested

When we're called with no hunk or line callback, we don't need to
do the hunk or line computation.

Microsoft is sponsoring a Pro account at AppVeyor for the libgit2
and LibGit2Sharp projects.  Point to that account's badge.

Remove strlen() calls from loop condition

Avoid str length recalculation every iteration

Add option to turn off OpenSSL

git_merge_tree_flag_t cannot contain any GIT_DIFF_FIND_xxx flags so there's not point in checking for them

The implementation of the hashsig API disallows computing a signature on
small files containing only a few lines. This new flag disables this
behavior.

git_diff_find_similar() sets this flag by default which means that rename
/ copy detection of small files will now work. This in turn affects the
behavior of the git_status and git_blame APIs which will now detect rename
of small files assuming the right options are passed.

Use svg instead of png to get better image quality

clone example: don't divide by zero

Local transports don't have data about the size, avoid dividing by
zero in the callback.

This should provide a easier way to see what kinds of changes we have,
and a single place to look at the breaking changes.

  Fix crash in free() when git_buf_grow() fails.