odb: consistently use "dir" to refer to alternate's directory

odb: consistently use "dir" to refer to alternate's directory

The functions that add an alternate object directory to the object
database are somewhat inconsistent in how they call the paramater that
refers to the directory path: in our headers we refer to it as "dir",
whereas in the implementation we often call it "reference" or "entry".

Unify this and consistently call the parameter "dir". While at it,
refactor `link_alt_odb_entry()` to accept a C string instead of a
`struct strbuf` as parameter to clarify that we really only need the
path and nothing else.

Suggested-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>