gitconfig: remove lint and diff5 aliases
gitconfig: remove init.templatedir

I only have this directory at work, so in my normal config, I should just use
the default value.
gitconfig: remove recent-branches.since
git: add fivebase alias; improve diff5

The 'fivebase' does the same as $(git5 status --base) but a hell of a lot
faster, and with an optional branch name parameter.  'diff5' now uses this,
which makes it much faster and also more useful.  For example, I can now 'git
diff5 <branch>', which I could not do before.
zshrc: add pager_if_tty
vimrc: remove gitgutter sign colors, update line colors

The latest version of gitgutter automatically uses the right sign column
background color so we don't need to define it ourselves.  The line colors are
still too bright (those of diff) so we now use some very subtle line colors
that match our background color.

Here is the thread describing the bug referenced in the comment:
https://www.mail-archive.com/vim_use@googlegroups.com/msg45293.html
vim: fix gitgutter \gl mapping
vim: add \gh / \gi to swap gitgutter diffbase

\gh switches to diffing from HEAD, while \gi switches to diffing from the
index.
vim: make sign column much more subtle

The old color make it stand out, which I found distracting.  This new color is
only very subtly darker than the regular background color.