checkout: provide better conflict hunk description with detached HEAD

When running 'git checkout -m' and using diff3 style conflict markers,
we want all the conflict hunks (left-side, "common" or "merge base", and
right-side) to have label markers letting the user know where each came
from.  The "common" hunk label (o.ancestor) came from
old_branch_info->name, but that is NULL when HEAD is detached, which
resulted in a blank label.  Check for that case and provide an
abbreviated commit hash instead.

(Incidentally, this was the only case in the git codebase where
merge_trees() was called with opt->ancestor being NULL.  A subsequent
commit will prevent similar problems by enforcing that merge_trees()
always be called with opt->ancestor != NULL.)

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/checkout.c

@@ -713,6 +713,7 @@ static int merge_working_tree(const struct checkout_opts *opts,
 			struct tree *old_tree;
 			struct merge_options o;
 			struct strbuf sb = STRBUF_INIT;
+			struct strbuf old_commit_shortname = STRBUF_INIT;
 
 			if (!opts->merge)
 				return 1;
@@ -768,6 +769,12 @@ static int merge_working_tree(const struct checkout_opts *opts,
 			if (ret)
 				return ret;
 			o.ancestor = old_branch_info->name;
+			if (old_branch_info->name == NULL) {
+				strbuf_add_unique_abbrev(&old_commit_shortname,
+							 &old_branch_info->commit->object.oid,
+							 DEFAULT_ABBREV);
+				o.ancestor = old_commit_shortname.buf;
+			}
 			o.branch1 = new_branch_info->name;
 			o.branch2 = "local";
 			ret = merge_trees(&o,
@@ -781,6 +788,7 @@ static int merge_working_tree(const struct checkout_opts *opts,
 					 opts, 0,
 					 writeout_error);
 			strbuf_release(&o.obuf);
+			strbuf_release(&old_commit_shortname);
 			if (ret)
 				return ret;
 		}