git-p4: support explicit sync of arbitrary existing git-p4 refs

git-p4: support explicit sync of arbitrary existing git-p4 refs

With the --branch argument of the "sync" subcommand, git-p4 enables
you to import a perforce branch/path to an arbitrary git ref, using
a full ref path, or to refs/remotes/p4/* or refs/heads/p4/*,
depending on --import-local, using a short ref name.

However, when you later want to explicitly sync such a given ref to
pick up subsequent p4 changes, it only works if the ref was placed
in the p4 path *and* has only one path component (no "/").

This limitation results from a bad assumption in the
existing-branch sync logic, and also means you cannot individually
sync branches detected by --detect-branches, as these also get a
"/" in their names.

Fix "git p4 sync --branch", when called with an existing ref, so
that it works correctly regardless of whether the ref is in the p4
path or not, and (in the case of refs in the p4 path) regardless of
whether it has a "/" in its short name or not.

Also add tests to validate that these branch-specific syncs work
as expected.

Signed-off-by: Tao Klerks <tao@klerks.biz>
Signed-off-by: Junio C Hamano <gitster@pobox.com>