fix bg bleed in git diff for whitespace errors

Alter the end pattern for the Bg syn regions in AnsiEsc to also match
the pattern git uses to reset ANSI colors, namely \e\[m .

There is still an issue with vim's conceal regions screwing up how many
spaces vim thinks is a tab, not sure how to fix this in AnsiEsc, we make
vimpager use a crude hack to work around this: force a :retab.