largefiles: make update with backup files in .hglf slightly less broken

Largefiles update would try to copy f to f.orig if there was a .hglf/f.orig .
That is in many many ways very very wrong, but it also caused an abort if f
didn't exist.

Now it only tries to copy f if it exists.

--HG--
branch : stable
convert: correct 'hooks' section name in online help

The section name for hooks is not 'hook', but 'hooks'.

--HG--
branch : stable
extra : rebase_source : 6c866a261c948b84cf0940b3b0caa3af9538c2b5
histedit: correct changeset IDs in online help

There is no '633536316234' revision in the example repository. It
should be 'c561b4e977df', according to the revisions in it and
explanation in help document.

--HG--
branch : stable
extra : rebase_source : 9f51e27795d615835924eb639ac1accaeb92444f
histedit: correct the number of added revisions in online help

In the context of help document on which this patch focuses, the
example repository, which is source of cloning, should be already
histedit-ed, and contain only 3 revisions (rev # 0 to 2). So, not 3,
but 4 revisions should be added to the destination repository of
cloning, if it contains 7 revisions (rev # 0 to 6).

This patch also adds modifier "histedit-ed" to "example repository",
to make context clear.

--HG--
branch : stable
extra : rebase_source : ee7daf86b6250a808e2d6a87a8a23a54c6fa4e49