Split validate_input subroutine into validate_pathname which is used
for $project, $file_name and $file_parent parameters, and
validate_refname which is used for $hash, $hash_base, $hash_parent and
$hash_parent_base parameters.  Reintroduce validation of $file_name
and $file_parent parameters, removed in a2f3db2f

validate_pathname in addition to what validate_input did checks also
for doubled slashes and NUL character. It does not check if input is
textual hash, and does not check if all characters are from the
following set: [a-zA-Z0-9_\x80-\xff\ \t\.\/\-\+\#\~\%].

validate_refname first check if the input is textual hash, then checks
if it is valid pathname, then checks for invalid characters (according
to git-check-ref-format manpage). It does not check if all charactes
are from the [a-zA-Z0-9_\x80-\xff\ \t\.\/\-\+\#\~\%] set.

We do not have to validate pathnames we got from git.
Signed-off-by: NJakub Narebski <jnareb@gmail.com>
Signed-off-by: NJunio C Hamano <junkio@cox.net>