completion: add deprecated __git_complete_file ()

77c130 (completion: clarify ls-tree, archive, show completion,
2013-06-02) removed __git_complete_file () because it had no callers
left in the file.  However, to avoid breaking user scripts that may
depend on this, add it back as a deprecated alias.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Revert 77c1305 and 3c3b46b

As explained by SZEDER Gábor in $gmane/226272, git_complete_file
helper is about completing <tree-ish>, taking <commit> at the tips
of refs and also understanding <tree-ish>:<path> notation, and
changing "archive" and "ls-tree" to use git_complete_revlist_file
that in addition is meant to expand A..B range notation was a
mistake.

Signed-off-by: Junio C Hamano <gitster@pobox.com>