strbuf: convert strbuf_add_unique_abbrev to use struct object_id

strbuf: convert strbuf_add_unique_abbrev to use struct object_id

Convert the declaration and definition of strbuf_add_unique_abbrev to
make it take a pointer to struct object_id.  Predeclare the struct in
strbuf.h, as cache.h includes strbuf.h before it declares the struct,
and otherwise the struct declaration would have the wrong scope.

Apply the following semantic patch, along with the standard object_id
transforms, to adjust the callers:

@@
expression E1, E2, E3;
@@
- strbuf_add_unique_abbrev(E1, E2.hash, E3);
+ strbuf_add_unique_abbrev(E1, &E2, E3);

@@
expression E1, E2, E3;
@@
- strbuf_add_unique_abbrev(E1, E2->hash, E3);
+ strbuf_add_unique_abbrev(E1, E2, E3);

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>