Use GIT_REFLOG_ACTION environment variable instead.

Use GIT_REFLOG_ACTION environment variable instead.

Junio rightly pointed out that the --reflog-action parameter
was starting to get out of control, as most porcelain code
needed to hand it to other porcelain and plumbing alike to
ensure the reflog contained the top-level user action and
not the lower-level actions it invoked.

At Junio's suggestion we are introducing the new set_reflog_action
function to all shell scripts, allowing them to declare early on
what their default reflog name should be, but this setting only
takes effect if the caller has not already set the GIT_REFLOG_ACTION
environment variable.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>