Add information on overriding environment variables using ~/.vimrc.
Work around unfortunate interaction between fugitive and :DiffGitCached.

Fugitive doesn't like having a :DiffGitCached command run for the
:Gstatus and :Gcommit windows.  The work-around in this commit
distinguishes between fugitive operations and editing a commit message
from "git commit" launched from the shell by checking the number of open
Vim windows.  When launched from "git commit", only one window will be
open; when using :Gstatus or :Gcommit, at least two will be open.

There may be a better solution.  A request has been opened with Tim
Pope, the author of the fugitive plugin, to see if he has any better
ideas:
https://github.com/tpope/vim-fugitive/issues/294