dir: consolidate treat_path() and treat_one_path()

dir: consolidate treat_path() and treat_one_path()

Commit 16e2cfa90993 ("read_directory(): further split treat_path()",
2010-01-08) split treat_one_path() out of treat_path(), because
treat_leading_path() would not have access to a dirent but wanted to
re-use as much of treat_path() as possible.  Not re-using all of
treat_path() caused other bugs, as noted in commit b9670c1f5e6b ("dir:
fix checks on common prefix directory", 2019-12-19).  Finally, in commit
ad6f2157f951 ("dir: restructure in a way to avoid passing around a
struct dirent", 2020-01-16), dirents were removed from treat_path() and
other functions entirely.

Since the only reason for splitting these functions was the lack of a
dirent -- which no longer applies to either function -- and since the
split caused problems in the past resulting in us not using
treat_one_path() separately anymore, just undo the split.

Signed-off-by: Elijah Newren <redacted>
Signed-off-by: Junio C Hamano <redacted>