To resolve a corner case uncovered by Simon Hausmann I need to
reuse the logic for the SHA-1 expression version of the 'from '
command within the mark version of the 'from ' command.  This change
doesn't alter any functionality, but is merely breaking the common
code out to a function that I can reuse.
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>