.gitignore: stop ignoring `.manifest` files

.gitignore: stop ignoring `.manifest` files

On Windows, it is possible to embed additional metadata into an
executable by linking in a "manifest", i.e. an XML document that
describes capabilities and requirements (such as minimum or maximum
Windows version). These XML documents are expected to be stored in
`.manifest` files.

At least _some_ Visual Studio versions auto-generate `.manifest` files
when none is specified explicitly, therefore we used to ask Git to
ignore them.

However, we do have a beautiful `.manifest` file now:
`compat/win32/git.manifest`, so neither does Visual Studio auto-generate
a manifest for us, nor do we want Git to ignore the `.manifest` files
anymore.

Further reading on auto-generated `.manifest` files:
https://docs.microsoft.com/en-us/cpp/build/manifest-generation-in-visual-studio

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>