cache.h: complete set of git_path_submodule helpers
The git_path function has "git_pathdup" and
"strbuf_git_path" variants, but git_submodule_path only
comes in the dangerous, static-buffer variant. That makes
refactoring callers to use the safer functions hard (since
they don't exist).
Since we're already using a strbuf behind the scenes, it's
easy to expose all three of these interfaces with thin
wrappers.
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>