Merge branch 'po/doc-assume-unchanged'

Fixes long-standing misunderstanding of what assume-unchanged is
about.  Some text near what is removed by the bottom patch may also
have to be removed.

* po/doc-assume-unchanged:
gitignore.txt: do not suggest assume-unchanged
doc: make clear --assume-unchanged's user contract

diff --cc Documentation/git-update-index.txt
Simple merge

diff --cc Documentation/gitignore.txt
Simple merge