revset: extract parsing logic out of formatspec

We want to be able to perform better handling of some input when running
revset (eg: `repo.revs("%ld", somerevs)`). The first step is to be able to
access some of the parsed content before it gets substituted. There are many
possible different substitutions, we'll add support for them gradually.

In this changeset we support none, we just split some logic in a sub function
as a preparatory step.

mercurial/revsetlang.py

@@ -666,6 +666,23 b' def formatspec(expr, *args):'
     >>> formatspec(b'%ls', [b'a', b"'"])
     "_list('a\\\\x00\\\\'')"
     '''
+    parsed = _parseargs(expr, args)
+    ret = []
+    for t, arg in parsed:
+        if t is None:
+            ret.append(arg)
+        else:
+            raise error.ProgrammingError("unknown revspec item type: %r" % t)
+    return b''.join(ret)
+
+def _parseargs(expr, args):
+    """parse the expression and replace all inexpensive args
+
+    return a list of tuple [(arg-type, arg-value)]
+
+    Arg-type can be:
+    * None: a string ready to be concatenated into a final spec
+    """
     expr = pycompat.bytestr(expr)
     argiter = iter(args)
     ret = []
@@ -673,16 +690,16 b' def formatspec(expr, *args):'
     while pos < len(expr):
         q = expr.find('%', pos)
         if q < 0:
-            ret.append(expr[pos:])
+            ret.append((None, expr[pos:]))
             break
-        ret.append(expr[pos:q])
+        ret.append((None, expr[pos:q]))
         pos = q + 1
         try:
             d = expr[pos]
         except IndexError:
             raise error.ParseError(_('incomplete revspec format character'))
         if d == '%':
-            ret.append(d)
+            ret.append((None, d))
             pos += 1
             continue
 
@@ -692,19 +709,20 b' def formatspec(expr, *args):'
             raise error.ParseError(_('missing argument for revspec'))
         f = _formatlistfuncs.get(d)
         if f:
-            # a list of some type
+            # a list of some type, might be expensive, do not replace
             pos += 1
             try:
                 d = expr[pos]
             except IndexError:
                 raise error.ParseError(_('incomplete revspec format character'))
             try:
-                ret.append(f(list(arg), d))
+                ret.append((None, f(list(arg), d)))
             except (TypeError, ValueError):
                 raise error.ParseError(_('invalid argument for revspec'))
         else:
+            # a single entry, not expensive, replace
             try:
-                ret.append(_formatargtype(d, arg))
+                ret.append((None, _formatargtype(d, arg)))
             except (TypeError, ValueError):
                 raise error.ParseError(_('invalid argument for revspec'))
         pos += 1
@@ -714,7 +732,7 b' def formatspec(expr, *args):'
         raise error.ParseError(_('too many revspec arguments specified'))
     except StopIteration:
         pass
-    return ''.join(ret)
+    return ret
 
 def prettyformat(tree):
     return parser.prettyformat(tree, ('string', 'symbol'))