Uses the usual completion mechanism of magit for gitignore.
Cleans up the ignore code containing redundant code and doc.
Use a default value when asking for a file to ignore
Put the submodule command back on a key.
Updated the documentation to reflect the new submobule key.
Merge commit '39939691e8cbc16aae75ac9b0d2b76aea5cb2fd6' into dudebout/submodules
Merge branch 'dudebout/submodules' into maint