Git: Make mergetool parsing more robust

Symbolic link conflict is printed in 2 phases: First "  {remote}", and
then "a symbolic link -> 'foo.cpp'".

This happens because this message involves an additional cat process:

  elif is_symlink "$mode"
  then
      echo "a symbolic link -> '$(cat "$file")'"

For local/remote line, wait for a full line before parsing.

Change-Id: I641cde12aa44dee2f7dff8fdae70da0c443e55d3
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/git/mergetool.h

@@ -82,6 +82,7 @@ private:
     QString m_localInfo;
     FileState m_remoteState = UnknownState;
     QString m_remoteInfo;
+    QByteArray m_line;
     bool m_merging = false;
 };