Backup g:agformat when changing it

Otherwise running :AgFile followed by :Ag will lead to results that
can't be jumped to.
typo
Merge pull request #64 from fernandoacorreia/patch-1

typo
Merge pull request #60 from losingkeys/agfile-breaks-other-searches

Backup g:agformat when changing it
Merge remote-tracking branch 'upstream/master'