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.

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.

Remove symlinks from the repository

remote: plug leak

Remove the hook symlink from the test resources, so that we can
have a source tree that is easy to zip up and copy around on systems
that don't support symlinks.  Create it dynamically at test execution
instead.

Load prune configuration when a remote is created.

CHANGELOG: we've added git_describe

submodule: declare vars at top of func block

Add appveyor yaml

CHANGELOG: add missing 0.22 changes

Better document `git_merge_commits`

The release after 0.21 is 0.22. Add a new heading for the changes since
0.22.