builtin/mv: convert assert(3p) into BUG()

The use of asserts is discouraged in our codebase because they lead to
different behaviour depending on how Git is built. When being unsure
enough whether a condition always holds so that one adds the assert,
then the assert should probably trigger regardless of how Git is being
built.

Drop the call to assert(3p) in git-mv(1) and instead use `BUG()`.

Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t

builtin/mv.c

@@ -560,7 +560,8 @@ int cmd_mv(int argc,
 			continue;
 
 		pos = index_name_pos(the_repository->index, src, strlen(src));
-		assert(pos >= 0);
+		if (pos < 0)
+			BUG("could not find source in index: '%s'", src);
 		if (!(mode & SPARSE) && !lstat(src, &st))
 			sparse_and_dirty = ie_modified(the_repository->index,
 						       the_repository->index->cache[pos],