index_init_entry() renamed to index_entry_init(). Now it allocates entry
on its own.

git_index_add() and git_index_append() reworked accordingly.

This commit fixes warning:

/home/kas/git/public/libgit2/src/index.c: In function ‘index_init_entry’:
/home/kas/git/public/libgit2/src/index.c:452:14: warning: assignment discards ‘const’ qualifier from pointer target type [enabled by default]

Signed-off-by: Kirill A. Shutemov <kirill@shutemov.name>