merge: make usage of commit->util more extensible

merge: make usage of commit->util more extensible

The merge-recursive code uses the commit->util field directly to annotate
the commit objects given from the command line, i.e. the remote heads to
be merged, with a single string to be used to describe it in its trace
messages and conflict markers.

Correct this short-signtedness by redefining the field to be a pointer to
a structure "struct merge_remote_desc" that later enhancements can add
more information. Store the original objects we were told to merge in a
field "obj" in this struct, so that we can recover the tag we were told to
merge.

Signed-off-by: Junio C Hamano <gitster@pobox.com>