revwalk: detect when we're out of interesting commits

Handle hide-then-push in the revwalk

Add memory leak detection/reporting using MSVC CRTDBG facility.

When walking backwards and marking parents uninteresting, make sure we
detect when the list of commits we have left has run out of
uninteresting commits so we can stop marking commits as
uninteresting. Failing to do so can mean that we walk the whole history
marking everything uninteresting, which eats up time, CPU and IO for
with useless work.

While pre-marking does look for this, we still need to check during the
main traversal as there are setups for which pre-marking does not leave
enough information in the commits. This can happen if we push a commit
and hide its parent.

As it seems it's not quite clear what it means to push a commit, try to
be more explicit about adding a new root and that we may not see this
commit if it is hidden.

Make sure to pack referenced objects for non-branches

If work_dir is not specified, use repo_dir to test if symlink is supported

Fix checking of return value for regcomp.

The regcomp function returns a non-zero value if compilation of
a regular expression fails. In most places we only check for
negative values, but positive values indicate an error, as well.
Fix this tree-wide, fixing a segmentation fault when calling
git_config_iterator_glob_new with an invalid regexp.

When a commit is first set as unintersting and then pushed, we must take
care that we do not put it into the commit list as that makes us return
at least that commit (but maybe more) as we've inserted it into the list
because we have the assumption that we want anything in the commit list.

When we hide a commit which we later push into the revwalk, we do not
handle this well and return commits which we should not.

Fix describe without refs and OID fallbacks

GIT_UNUSED() macro generates code therefore it should be used after
variable declarations.

When no reference names could be found we did error out when trying to describe
a commit. This is wrong, though, when the option to fall back to a commit's
object ID is set.

Entry argument passed to git_index_add_frombuffer() should be const

gitignore: ignore vim swapfiles

Clear temporary buffer when filtering

If we are using a temporary buffer for filtering, be sure to clear
it before using it, in case the file that we are filtering is empty.

squash some leaks

Add a custom param to git_smart_subtransport_definition

WinHTTP for MinGW

note: use a git_buf to return the default namespace

examples: add remote example.

checkout: report correct invalid path

In case of a bad url or other error during the connection setup, we
close the stream via free.

fix changelog

mkdir-ext: Assume directories don't exist; fix all race cases