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

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: Johannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>