refs.c: do not die if locking fails in `write_pseudoref()`

refs.c: do not die if locking fails in `write_pseudoref()`

If we could not take the lock, we add an error to the `strbuf err` and
return. However, this code is dead. The reason is that we take the lock
using `LOCK_DIE_ON_ERROR`. Drop the flag to allow our more gentle
error-handling to actually kick in.

We could instead just drop the dead code and die here. But everything is
prepared for gently propagating the error, so let's do that instead.

There is similar dead code in `delete_pseudoref()`, but let's save that
for the next patch.

While at it, make the lock non-static. (Placing `struct lock_file`s on
the stack used to be a bad idea, because the temp- and
lockfile-machinery would keep a pointer into the struct. But after
076aa2cbd (tempfile: auto-allocate tempfiles on heap, 2017-09-05), we
can safely have lockfiles on the stack.)

Reviewed-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>