the newly constructed pack and index before refs can be
updated to use objects contained in the pack.
---keep='why'::
+--keep=<msg>::
Like --keep create a .keep file before moving the index into
its final destination, but rather than creating an empty file
- place 'why' followed by an LF into the .keep file. The 'why'
+ place '<msg>' followed by an LF into the .keep file. The '<msg>'
message can later be searched for within all .keep files to
locate any which have outlived their usefulness.