path.c: add git_common_path() and strbuf_git_common_path()

path.c: add git_common_path() and strbuf_git_common_path()

These are mostly convenient functions to reduce code duplication. Most
of the time, we should be able to get by with git_path() which handles
$GIT_COMMON_DIR internally. However there are a few cases where we need
to construct paths manually, for example some paths from a specific
worktree. These functions will enable that.

Signed-off-by: Nguyễn Thái Ngọc Duy <redacted>
Signed-off-by: Junio C Hamano <redacted>