magit-grep: highlight match

This command is no good, and this change makes little difference.  We
could make proper use of `git grep' and that would be a great feature,
but as of now this is just a severely crippled version of `rgrep'.

Suggested-by: Björn Lindqvist <bjourne@gmail.com>
Fix a few minor spelling errors in doc strings.
fix missing back-tick quoting of keys in doc strings
magit-run-git-gui-blame: let user choose filename when necessary
merge from origin/master

(with minor re-fix to doc-string for magit-run-git-gui-blame)
document magit-rename-item
correct documentation string for magit-stash
fix documentation string for magit-cherry-pick-item