$ git status
[...]
# Untracked files:
[...]
# Documentation/foo.html
# Documentation/gitignore.html
# file.o
# lib.a
# src/internal.o
[...]
$ cat .git/info/exclude
# ignore objects and archives, anywhere in the tree.
*.[oa]
$ cat Documentation/.gitignore
# ignore generated html files,
*.html
# except foo.html which is maintained by hand
!foo.html
$ git status
[...]
# Untracked files:
[...]
# Documentation/foo.html
[...]
Another example:
$ cat .gitignore
vmlinux*
$ ls arch/foo/kernel/vm*
arch/foo/kernel/vmlinux.lds.S
$ echo '!/vmlinux*'