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 <spearce@spearce.org>

lib/merge.tcl

@@ -90,9 +90,6 @@ method _start {} {
 
 	set spec [$w_rev get_tracking_branch]
 	set cmit [$w_rev get_commit]
-	set cmd [list git]
-	lappend cmd merge
-	lappend cmd --strategy=recursive
 
 	set fh [open [gitdir FETCH_HEAD] w]
 	fconfigure $fh -translation lf
@@ -112,6 +109,9 @@ method _start {} {
 	puts $fh "$cmit\t\tbranch '$branch' of $remote"
 	close $fh
 
+	set cmd [list git]
+	lappend cmd merge
+	lappend cmd --strategy=recursive
 	lappend cmd [git fmt-merge-msg <[gitdir FETCH_HEAD]]
 	lappend cmd HEAD
 	lappend cmd $cmit