rebase -i: clarify in-editor documentation of "exec"

rebase -i: clarify in-editor documentation of "exec"

The hints in the current "instruction sheet" template look like so:

 # Rebase 3f14246..a1d7e01 onto 3f14246
 #
 # Commands:
 #  p, pick = use commit
 #  r, reword = use commit, but edit the commit message
 #  e, edit = use commit, but stop for amending
 #  s, squash = use commit, but meld into previous commit
 #  f, fixup = like "squash", but discard this commit's log message
 #  x <cmd>, exec <cmd> = Run a shell command <cmd>, and stop if it fails
 #
 # If you remove a line here THAT COMMIT WILL BE LOST.
 # However, if you remove everything, the rebase will be aborted.
 #

This does not make it clear that the format of each line is

	<insn> <commit id> <explanatory text that will be printed>

but the reader will probably infer that from the automatically
generated pick examples above it.

What about the "exec" instruction?  By analogy, I might imagine that
the format of that line is "exec <command> <explanatory text>", and
the "x <cmd>" hint does not address that question (at first I read it
as taking an argument <cmd> that is the name of a shell).  Meanwhile,
the mention of <cmd> makes the hints harder to scan as a table.

So remove the <cmd> and add some words to remind the reader that
"exec" runs a command named by the rest of the line.  To make room, it
is left to the manpage to explain that that command is run using
$SHELL and that nonzero status from that command will pause the
rebase.

Wording from Junio.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>