git-gui: Do not munge conflict marker lines in a normal diff

Previously, conflict markers were highlighted in two ways: (1) They
received a distinguishing color; and (2) they had the '+' removed at the
beginning of the line. However, by doing (2), a hunk that contained
conflict markers could not be staged or unstaged because the resulting
patch was corrupted. With this change we no longer modify the diff text
of a 2-way diff, so that "Stage Hunk" and friends work.

Note that 3-way diff of a conflicted file is unaffected by this change,
and '++' before conflict markers is still removed. But this has no negative
impact because in this mode staging hunks or lines is disabled anyway.
Signed-off-by: NJohannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>

lib/diff.tcl

@@ -377,7 +377,6 @@ proc read_diff {fd cont_info} {
 			{+} {
 				if {[regexp {^\+([<>]{7} |={7})} $line _g op]} {
 					set is_conflict_diff 1
-					set line [string replace $line 0 0 { }]
 					set tags d$op
 				} else {
 					set tags d_+