path: extract format_path() and use in rev-parse

path: extract format_path() and use in rev-parse

Path formatting logic in builtin/rev-parse.c writes directly to
stdout. Other builtins cannot reuse it.

Extract this logic into format_path() in path.c and expose
a path_format enum in path.h.

Convert rev-parse to use the new helper in the same step to validate
the API against existing tests and avoid introducing dead code.

Mentored-by: Justin Tobler <redacted>
Mentored-by: Lucas Seiki Oshiro <redacted>
Signed-off-by: K Jayatheerth <redacted>
Signed-off-by: Junio C Hamano <redacted>