message: Only update save time headers on explicit save
  Don't update PO-Revision-Date and/or POT-Creation-Date in DoSaveOnly,
  which is used internally in various places to dump the data into a file
  for further processing. Instead, update the headers only in
  Catalog::Save() which is used when the user explicitly saves a file.
  Fixes
diff: @@ -1553,6 +1553,23 @@ bool Catalog::Save(const wxString& po_file, bool save_mo,
  return false;
  }
  + // Update information about last modification time. But if the header
  + // was empty previously, the author apparently doesn't want this header
  + // set, so don't mess with it. See https://sourceforge.net/tracker/?func=detail&atid=389156&aid=1900298&group_id=27043
  + // for motivation:
  + auto currentTime = GetCurrentTimeString();
  + switch (m_fileType)
  + {
  + case Type::PO:
  + if ( !m_header.RevisionDate.empty() )
  + m_header.RevisionDate = currentTime;
  + break;
  + case Type::POT:
  + if ( m_fileType == Type::POT && !m_header.CreationDate.empty() )
  + m_header.CreationDate = currentTime;
  + break;
  + }
  +
  TempOutputFileFor po_file_temp_obj(po_file);
  const wxString po_file_temp = po_file_temp_obj.FileName();
  @@ -1850,23 +1867,6 @@ bool Catalog::DoSaveOnly(wxTextBuffer& f, wxTextFileType crlf)
  if (!m_header.Charset || m_header.Charset == "CHARSET")
  m_header.Charset = "UTF-8";
  - // Update information about last modification time. But if the header
  - // was empty previously, the author apparently doesn't want this header
  - // set, so don't mess with it. See https://sourceforge.net/tracker/?func=detail&atid=389156&aid=1900298&group_id=27043
  - // for motivation:
  - auto currentTime = GetCurrentTimeString();
  - switch (m_fileType)
  - {
  - case Type::PO:
  - if ( !m_header.RevisionDate.empty() )
  - m_header.RevisionDate = currentTime;
  - break;
  - case Type::POT:
  - if ( m_fileType == Type::POT && !m_header.CreationDate.empty() )
  - m_header.CreationDate = currentTime;
  - break;
  - }
  -
  SaveMultiLines(f, m_header.Comment);
  if (m_fileType == Type::POT)
  f.AddLine(wxS("#, fuzzy"));
  