Merge branch 'jk/remote-helpers-doc'

"git help remote-helpers" did not work; 'remote-helpers' is not
a subcommand name but a concept, so its documentation should have
been in gitremote-helpers, not git-remote-helpers.

* jk/remote-helpers-doc:
  Rename {git- => git}remote-helpers.txt

diff --cc Documentation/Makefile
Simple merge

diff --cc Documentation/git-remote-testgit.txt
Simple merge

diff --cc Documentation/gitremote-helpers.txt
Simple merge

diff --cc Documentation/urls.txt
Simple merge