Have init script treat master unambiguously as a branch

Because users may have an old version of git without "git switch",
init-tests-after-clone.sh should continue to use "git checkout" to
attempt to switch to master. But without "--", this suffers from
the problem that it's ambiguous if master is a branch (so checkout
behaves like switch) or a path (so checkout behaves like restore).

There are two cases where this ambiguity can be a problem. The most
common is on a fork with no master branch but also, fortunately, no
file or directory named "master". Then the problem is just the
error message (printed just before the script proceeds to redo
the checkout with -b):

    error: pathspec 'master' did not match any file(s) known to git

The real cause of the error is the branch being absent, as happens
when a fork copies only the main branch and the upstream remote is
not also set up. Adding the "--" improves the error message:

    fatal: invalid reference: master

However, it is possible, though unlikely, for a file or directory
called "master" to exist. In that case, if there is also no master
branch, git discards unstaged changes made to the file or (much
worse!) everywhere in that directory, potentially losing work.

This commit adds "--" to the right of "master" so git never
regards it as a path. This is not needed with -b, which is always
followed by a symbolic name, so I have not added it there.

(Note that the command is still imperfect because, for example, in
rare cases there could be a master *tag*--and no master branch--in
which case, as before, HEAD would be detached there and the script
would attempt to continue.)

init-tests-after-clone.sh

@@ -10,7 +10,7 @@ if [[ -z "$TRAVIS" ]]; then
 fi
 
 git tag __testing_point__
-git checkout master || git checkout -b master
+git checkout master -- || git checkout -b master
 git reset --hard HEAD~1
 git reset --hard HEAD~1
 git reset --hard HEAD~1