Disambiguate the reset and reset_range functions.  Now reset_range
with a NULL path will clear the start or end; reset will leave the
existing start and end unchanged.

More coverity fixes

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.

We cannot know from looking at .gitmodules whether a directory is a
submodule or not. We need the index or tree we are comparing against to
tell us. Otherwise we have to assume the entry in .gitmodules is stale
or otherwise invalid.

Thus we pass the index of the repository into the workdir iterator, even
if we do not want to compare against it. This follows what git does,
which even for `git diff <tree>`, it will consider staged submodules as
such.

Only apply LEADING_DIR pattern munging to patterns in ignore and
attribute files, not to pathspecs used to select files to operate
on.  Also, allow internal macro definitions to be evaluated before
loading all external ones (important so that external ones can
make use of internal `binary` definition).

Clear up some of the various "find" functions and the snapshot
API naming to be things I like more.