clone: rename static function `dir_exists()`.

clone: rename static function `dir_exists()`.

builtin/clone.c has a static function dir_exists() that
checks if a given path exists on the filesystem.  It returns
true (and it is correct for it to return true) when the
given path exists as a non-directory (e.g. a regular file).

This is confusing.  What the caller wants to check, and what
this function wants to return, is if the path exists, so
rename it to path_exists().

Signed-off-by: Miriam Rubio <redacted>
Signed-off-by: Junio C Hamano <redacted>