grep: give different labels to + and - symbols

I find it more useful to give different colours to plus and minus, but
it's difficult to do so if the default output uses the same label for
both. The following augments the names of columns with some extra
labels, akin to the diff.inserted and diff.deleted labels for the diff
command.

This is done by adding an extra label field to the columns tuples.

mercurial/commands.py

@@ -2885,21 +2885,25 b' def grep(ui, repo, pattern, *pats, **opt'
             fm.plain(uipathfn(fn), label='grep.filename')
 
             cols = [
-                ('rev', '%d', rev, not plaingrep),
-                ('linenumber', '%d', l.linenum, opts.get('line_number')),
+                ('rev', '%d', rev, not plaingrep, ''),
+                ('linenumber', '%d', l.linenum, opts.get('line_number'), ''),
             ]
             if diff:
-                cols.append(('change', '%s', change, True))
+                cols.append(
+                    ('change', '%s', change, True,
+                     'grep.inserted ' if change == '+' else 'grep.deleted ')
+                )
             cols.extend([
-                ('user', '%s', formatuser(ctx.user()), opts.get('user')),
+                ('user', '%s', formatuser(ctx.user()), opts.get('user'), ''),
                 ('date', '%s', fm.formatdate(ctx.date(), datefmt),
-                 opts.get('date')),
+                 opts.get('date'), ''),
             ])
-            for name, fmt, data, cond in cols:
+            for name, fmt, data, cond, extra_label in cols:
                 if cond:
                     fm.plain(sep, label='grep.sep')
                 field = fieldnamemap.get(name, name)
-                fm.condwrite(cond, field, fmt, data, label='grep.%s' % name)
+                label = extra_label + ('grep.%s' % name)
+                fm.condwrite(cond, field, fmt, data, label=label)
             if not opts.get('files_with_matches'):
                 fm.plain(sep, label='grep.sep')
                 if not opts.get('text') and binary():

tests/test-grep.t

@@ -299,6 +299,10 b' Test wdir'
   color:3:+:orange
   color:2:-:orange
   color:1:+:orange
+  $ hg grep --diff orange --color=debug
+  [grep.filename|color][grep.sep|:][grep.rev|3][grep.sep|:][grep.inserted grep.change|+][grep.sep|:][grep.match|orange]
+  [grep.filename|color][grep.sep|:][grep.rev|2][grep.sep|:][grep.deleted grep.change|-][grep.sep|:][grep.match|orange]
+  [grep.filename|color][grep.sep|:][grep.rev|1][grep.sep|:][grep.inserted grep.change|+][grep.sep|:][grep.match|orange]
 
   $ hg grep --diff orange
   color:3:+:orange