revset: add wdir() function to specify workingctx revision by command

The main purpose of wdir() is to annotate working-directory files.

Currently many commands and revsets cannot handle workingctx and may raise
exception. For example, -r ":wdir()" results in TypeError. This problem will
be addressed by future patches.

We could add "wdir" symbol instead, but it would conflict with the existing
tag, bookmark or branch. So I decided not to.

List of commands that will potentially support workingctx revision:

  command   default  remarks
  --------  -------  -----------------------------------------------------
  annotate  p1       useful
  archive   p1       might be useful
  cat       p1       might be useful on Windows (no cat)
  diff      p1:wdir  (default)
  export    p1       might be useful if wctx can have draft commit message
  files     wdir     (default)
  grep      tip:0    might be useful
  identify  wdir     (default)
  locate    wdir     (default)
  log       tip:0    might be useful with -p or -G option
  parents   wdir     (default)
  status    wdir     (default)

This patch includes minimal test of "hg status" that should be able to handle
the workingctx revision.

mercurial/revset.py

@@ -1852,6 +1852,16 b' def user(repo, subset, x):'
     """
     return author(repo, subset, x)
 
+def wdir(repo, subset, x):
+    """``wdir()``
+    Working directory.
+    """
+    # i18n: "wdir" is a keyword
+    getargs(x, 0, 0, _("wdir takes no arguments"))
+    if None in subset:
+        return baseset([None])
+    return baseset()
+
 # for internal use
 def _list(repo, subset, x):
     s = getstring(x, "internal error")
@@ -1947,6 +1957,7 b' symbols = {'
     "tagged": tagged,
     "user": user,
     "unstable": unstable,
+    "wdir": wdir,
     "_list": _list,
     "_intlist": _intlist,
     "_hexlist": _hexlist,
@@ -2019,6 +2030,7 b' safesymbols = set(['
     "tagged",
     "user",
     "unstable",
+    "wdir",
     "_list",
     "_intlist",
     "_hexlist",

tests/test-revset.t

@@ -509,6 +509,14 b' Test null revision'
   1
   0
 
+Test working-directory revision
+  $ hg debugrevspec 'wdir()'
+  None
+  $ hg debugrevspec 'tip or wdir()'
+  9
+  None
+  $ hg debugrevspec '0:tip and wdir()'
+
   $ log 'outgoing()'
   8
   9

tests/test-status.t

@@ -240,6 +240,17 b" Check 'status -q' and some combinations"
   $ rm deleted
   $ hg copy modified copied
 
+Specify working directory revision explicitly, that should be the same as
+"hg status"
+
+  $ hg status --change "wdir()"
+  M modified
+  A added
+  A copied
+  R removed
+  ! deleted
+  ? unknown
+
 Run status with 2 different flags.
 Check if result is the same or different.
 If result is not as expected, raise error