vcs-svn: Rename dirent pool to build on Windows

vcs-svn: Rename dirent pool to build on Windows

dirent is #define’d to mingw_dirent in compat/mingw.h, with the
result that

 obj_pool_gen(dirent, struct repo_dirent, 4096)

creates functions with names like mingw_dirent_alloc and
references to dirent_alloc go unresolved.  Rename the functions
to dent_* to avoid this problem.

Reported-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>