hg: fix userspec
git, hg: symlink configs
git: fix userspec for older versions of git

Turns out, support for include files is a very new feature of Git, so
have bot the default values for userspec an include for override, if
available.
vim: don't set textwidth, annoying
vim, hg: look up commit prefix by repo url

Have a dict with repo URLs mapping to this repo-specific commit prefix,
insert that prefix as the first line in buffer.