revset: use '%' as an operator for 'only'

With this patch, we can make it much easier to specify 'only(A,B)' ->
A%B. Similarly, 'only(A)' -> A%.

On Windows, '%' is a semi-reserved symbol in the following way: using non-bash
shells (e.g. cmd.exe but NOT PowerShell, ConEmu, and cmder), %var% is only
expanded when 'var' exists and is surrounded by '%'.

That only leaves batch scripts which could prove to be problematic. I posit
that this isn't a big issue because any developer of batch scripts already
knows that to use '%' one needs to escape it by using a double '%%'.

Alternatives to '%' could be '=' but that might be limiting our future if we
ever decide to use temporary assignments in a revset.

mercurial/revset.py

@@ -116,6 +116,7 b' elements = {'
     "!": (10, ("not", 10)),
     "and": (5, None, ("and", 5)),
     "&": (5, None, ("and", 5)),
+    "%": (5, None, ("only", 5), ("onlypost", 5)),
     "or": (4, None, ("or", 4)),
     "|": (4, None, ("or", 4)),
     "+": (4, None, ("or", 4)),
@@ -152,7 +153,7 b' def tokenize(program, lookup=None):'
         elif c == '#' and program[pos:pos + 2] == '##': # look ahead carefully
             yield ('##', None, pos)
             pos += 1 # skip ahead
-        elif c in "():,-|&+!~^": # handle simple operators
+        elif c in "():,-|&+!~^%": # handle simple operators
             yield (c, None, pos)
         elif (c in '"\'' or c == 'r' and
               program[pos:pos + 2] in ("r'", 'r"')): # handle quoted strings
@@ -1922,6 +1923,8 b' methods = {'
     "ancestor": ancestorspec,
     "parent": parentspec,
     "parentpost": p1,
+    "only": only,
+    "onlypost": only,
 }
 
 def optimize(x, small):
@@ -1935,6 +1938,9 b' def optimize(x, small):'
     op = x[0]
     if op == 'minus':
         return optimize(('and', x[1], ('not', x[2])), small)
+    elif op == 'only':
+        return optimize(('func', ('symbol', 'only'),
+                         ('list', x[1], x[2])), small)
     elif op == 'dagrangepre':
         return optimize(('func', ('symbol', 'ancestors'), x[1]), small)
     elif op == 'dagrangepost':

tests/test-revset.t

@@ -438,6 +438,32 b' Test empty set input'
   8
   9
 
+Test '%' operator
+
+  $ log '9%'
+  8
+  9
+  $ log '9%5'
+  2
+  4
+  8
+  9
+  $ log '(7 + 9)%(5 + 2)'
+  4
+  6
+  7
+  8
+  9
+
+Test the order of operations
+
+  $ log '7 + 9%5 + 2'
+  7
+  2
+  4
+  8
+  9
+
 Test explicit numeric revision
   $ log 'rev(-1)'
   $ log 'rev(0)'