util.checkcase: don't abort on broken symlinks
One case where that would happen is while trying to resolve a subrepo, if the
path to the subrepo was actually a broken symlink. This bug was exposed by an
hg-git test.
# The 9diff extdiff command generates diff(1) compatible output
# suitable for use with the plumber(4).
[extensions]
extdiff =
[extdiff]
9diff = /bin/mercurial/9diff -cm $parent $child $root