I propose the attached patch, which fixes the following problem:

1. M-x shell
2. grep -R -n --color=always 'some-search-string-which-yeilds-resuts'
   File names and line numbers output by grep should be in color
3. M-x compilation-shell-minor-mode
   File names and line numbers are now underlined by compile.el
4. M-x compilation-shell-minor-mode, to turn it back off

File names and line numbers lose their fontification completely, that
is, they are now in the default face.

The attached patch makes them return to the face they had after step 2,
as specified by the grep command. It also "name-spaces" other text
properties used by compile.el, such as keymap and mouse-face. If other
minor or major modes make use of these text properties, turning off
compilation-shell-minor-mode or compilation-minor-mode should now leave
them alone. (Though the ones from compile.el take precedence as long as
compilation-*-mode is active.)