git-gui: Minor refactoring of merge command line in merge support
git-gui: Minor refactoring of merge command line in merge support

This is just a small code movement to cleanup how we generate
the command line for a merge.  I'm only doing it to make the
next series of changes slightly more readable.

Signed-off-by: Shawn O. Pearce <f01032651c12a22b2103346ba96b3b6b231f9d3c@spearce.org>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git