Bump version to 0.21.4

A rule "src" in src/.gitignore must only match subdirectories of
src/. The current code does not include this context in the match rule
and would thus consider this rule to match the top-level src/ directory
instead of the intended src/src/.

Keep track fo the context in which the rule was defined so we can
perform a prefix match.

A rule can only negate something which was explicitly mentioned in the
rules before it. Change our parsing to ignore a negative rule which does
not negate something mentioned in the rules above it.

While here, fix a wrong allocator usage. The memory for the match string
comes from pool allocator. We must not free it with the general
allocator. We can instead simply forget the string and it will be
cleaned up.

Given

    top
    !top/foo

in an ignore file, we should not unignore top/foo. This is an
implementation detail of the git code leaking, but that's the behaviour
we should show.

A negation rule can only negate an exact rule it has seen before.

[maint/v0.21] Fix build of tests with mingw

very, very partial backport of 2f795d8fc50d81641d95723d9ddd92795886bed3 to maint/v0.21 branch

without this include, O_RDWR and O_CREAT are undeclared when building tests/path/win32.c with mingw

We need to know what wchar_t and MAX_PATH are. Including common.h takes
care of that for us.

GIT_BEGIN/END_DECL were missing from sys/refs.h and preventing
compilation with g++ as the symbol were mangled.