rename variables so regex/regexes is diff_regex/regex

rename variables so regex/regexes is diff_regex/regex

All the code to check for patterns within diff are named regex,
regexes, etc. This is problematic because (a) we already have one more
place we can check for regex and (b) I'm about to about add code to look
for regex in abitrary revisions (/not/ diffs). This change "makes room"
for the next one.