glossary: substitute "ancestor" for "direct ancestor" in 'push' description.

glossary: substitute "ancestor" for "direct ancestor" in 'push' description.

Even though "direct ancestor" is not defined in the glossary, the
common meaning of the term is simply "parent", parents being the only
direct ancestors, and the rest of ancestors being indirect ancestors.

As "parent" is obviously wrong in this place in the description, we
should simply say "ancestor", as everywhere else.

Signed-off-by: Sergey Organov <redacted>
Signed-off-by: Junio C Hamano <redacted>