launcher: simplify .repo search ceiling check

In the .repo discovery loop

  while curdir != '/' and curdir != olddir:
    ... break if we found .repo ...
    olddir = curdir
    curdir = os.path.dirname(curdir)

the "while" condition is meant to avoid searching forever if we do not
find .repo before reaching the top-level directory of the filesystem.
For that purpose, the first half of the condition is redundant; once
we reach "/", the parent directory will be "/" again and the curdir !=
olddir check would suffice to terminate the search.  Simplify by
removing the redundant first half of the check.

Noticed by code inspection.  The first half of the check was retained
when introducing the second half in df14a70c

 ("Make path references OS
independent", 2011-01-09), in an excess of caution.

This also improves consistency a little: if I start with curdir =
'/home/me', then with the redundant check in place we search

	/home/me
	/home

before hitting / and giving up.  On Windows, if I start with
'c:/users/me', then we search

	c:/users/me
	c:/users
	c:/

before hitting a repetition and giving up.  Fortunately it is not
common for people to set up repo clients at the top level of
filesystems, but consistently following the latter behavior should
make debugging a little easier in case it comes up.

Link: https://gerrit-review.googlesource.com/id/Ib9e830e3b9adfb1c4e56f3bcfba4746c401fb84f
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/286002
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Jonathan Nieder <jrn@google.com>