Documentation: How to ignore local changes in tracked files

Documentation: How to ignore local changes in tracked files

This patch explains more carefully that `.gitignore` concerns only
untracked files and refers the reader to

	git update-index --assume-unchanged

in the need of ignoring uncommitted changes in already tracked files.
The description of this option is lifted to a more "porcelainish"
level and explains the caveats of this usecase.

Whether feasible or not, I believe adding this functionality to
the porcelain is out of the scope of this patch. (And I personally
think that referring to the plumbing in the case of such a special
usage is fine.)

This is currently probably one of the top FAQs at #git and the
--assume-unchanged switch is not widely known; gitignore(5) is the first
place where people are likely to look for it.

Signed-off-by: Petr Baudis <pasky@suse.cz>
Signed-off-by: Junio C Hamano <gitster@pobox.com>