[PATCH] daemon.c and path.enter_repo(): revamp path validation.

The whitelist of git-daemon is checked against return value from
enter_repo(), and enter_repo() used to return the value obtained
from getcwd() to avoid directory aliasing issues as discussed
earier (mid October 2005).

Unfortunately, it did not go well as we hoped.

For example, /pub on a kernel.org public machine is a symlink to
its real mountpoint, and it is understandable that the
administrator does not want to adjust the whitelist every time
/pub needs to point at a different partition for storage
allcation or whatever reasons.  Being able to keep using
/pub/scm as the whitelist is a desirable property.

So this version of enter_repo() reports what it used to chdir()
and validate, but does not use getcwd() to canonicalize the
directory name.  When it sees a user relative path ~user/path,
it internally resolves it to try chdir() there, but it still
reports ~user/path (possibly after appending .git if allowed to
do so, in which case it would report ~user/path.git).

What this means is that if a whitelist wants to allow a user
relative path, it needs to say "~" (for all users) or list user
home directories like "~alice" "~bob".  And no, you cannot say
/home if the advertised way to access user home directories are
~alice,~bob, etc.  The whole point of this is to avoid
unnecessary aliasing issues.

Anyway, because of this, daemon needs to do a bit more work to
guard itself.  Namely, it needs to make sure that the accessor
does not try to exploit its leading path match rule by inserting
/../ in the middle or hanging /.. at the end.  I resurrected the
belts and suspender paranoia code HPA did for this purpose.

This check cannot be done in the enter_repo() unconditionally,
because there are valid callers of enter_repo() that want to
honor /../; authorized users coming over ssh to run send-pack
and fetch-pack should be allowed to do so.

Signed-off-by: Junio C Hamano <junkio@cox.net>

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