strvec: add functions to replace and remove strings

strvec: add functions to replace and remove strings

Add two functions that allow to replace and remove strings contained in
the strvec. This will be used by a subsequent commit that refactors
git-mv(1).

While at it, add a bunch of unit tests that cover both old and new
functionality.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>