This commit fixes highlighting the part of the line of code that matches
the git grep query in cases where the match starts at the beginning of
the lines.

Fixes dandavison#1056.

In some cases, `git grep` will customize the foreground-color for more
than just the subset of the line that matches the grep pattern. This
breaks the current match-detection behavior, which considers any
characters with a non-default "foreground color" within the "code" part
of a git-grep-ouput-line to be part of the match. This commit makes
match-detection check for boldness and a red foreground instead of just
checking for a non-default foreground-color.

git grep matches are bold and red by default. But git grep isn't the
only reason input to delta may contain color codes; there may still be
cases where the highlighting looks wrong here.