Rename {git- => git}remote-helpers.txt

Rename {git- => git}remote-helpers.txt

When looking up a topic via "git help <topic>", git-help prepends "git-"
to topics that are the names of commands (either builtin or found on the
path) and "git" (no hyphen) to any other topic name.

"git-remote-helpers" is not the name of a command, so "git help
remote-helpers" looks for "gitremote-helpers" and does not find it.

Fix this by renaming "git-remote-helpers.txt" to
"gitremote-helpers.txt".

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>