While C Git has been writing entry count -1 (ie. never other negative
numbers) as invalid since day 1, it accepts all negative entry counts
as invalid. JGit follows the same rule. libgit2 should also follow, or
the index that works with C Git or JGit may someday be rejected by
libgit2.

Other reimplementations like dulwich and grit have not bothered with
parsing or writing tree cache.

The code used to assume that there had to be data after the newline in
a tree cache extension entry. This isn't true for a childless
invalidated entry if it's the last one, as there won't be any children
nor a hash to take up space.

Adapt the off-by-one comparison to also work in this case. Fixes #633.

Signed-off-by: schu <schu-github@schulog.org>

Ensure that all memory related functions (malloc, calloc, strdup, free,
etc) are using their respective `git__` wrappers.

Signed-off-by: Carlos Martín Nieto <cmn@elego.de>

Whenever a file is updated in the index, each tree leading towards it
needs to be invalidated. Provide the supporting function.

Signed-off-by: Carlos Martín Nieto <cmn@elego.de>

The fix introduced in a02fc2cd1 (2011-05-24; index: correctly parse
invalidated TREE extensions) threw out the rest of the data in the
extension if it found an invalidated entry. This was the result of
incorrect reading of the documentation.

Insted, keep reading the extension, as there may be cached data we can
use.

Signed-off-by: Carlos Martín Nieto <cmn@elego.de>