grep: don't go in an infinite loop when given empty regex

Differential Revision: https://phab.mercurial-scm.org/D8543

mercurial/commands.py

@@ -3428,8 +3428,11 b' def grep(ui, repo, pattern, *pats, **opt'
                 m = regexp.search(self.line, p)
                 if not m:
                     break
-                yield m.span()
-                p = m.end()
+                if m.end() == p:
+                    p += 1
+                else:
+                    yield m.span()
+                    p = m.end()
 
     matches = {}
     copies = {}

tests/test-grep.t

@@ -660,7 +660,7 b' Existing tracked files in the working di'
   $ echo 'added, missing' > added-missing; hg add added-missing; rm added-missing
   $ echo 'untracked' > untracked
   $ hg rm old
-  $ hg grep '[^Z]'
+  $ hg grep ''
   added:added
   new:something else
   new:modified