cache.h: complete set of git_path_submodule helpers

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 <redacted>
Signed-off-by: Junio C Hamano <redacted>