config: introduce a new value for ui.relative-paths getting old behavior

The few places I've modified so far to respect ui.relative-paths have
traditionally defaulted showing the path from the repo root. However,
some commands (at least `hg files`) default to showing paths relative
to the cwd. Let's allow a special value for ui.relative-paths to
preserve the old behavior, so we can use that as default value for
it. I don't expect that anyone would want to set this value, so
perhaps we could have relied on it being unset, but I don't really
like behaviors that can only be achieved by a unset config option.

Differential Revision: https://phab.mercurial-scm.org/D5800

mercurial/commands.py

@@ -5421,7 +5421,7 b' def status(ui, repo, *pats, **opts):'
         relative = True
     elif ui.hasconfig('commands', 'status.relative'):
         relative = ui.configbool('commands', 'status.relative')
-    uipathfn = scmutil.getuipathfn(repo, relative)
+    uipathfn = scmutil.getuipathfn(repo, forcerelativevalue=relative)
 
     if opts.get('print0'):
         end = '\0'

mercurial/configitems.py

@@ -1207,7 +1207,7 b" coreconfigitem('ui', 'quietbookmarkmove'"
     default=False,
 )
 coreconfigitem('ui', 'relative-paths',
-    default=False,
+    default='legacy',
 )
 coreconfigitem('ui', 'remotecmd',
     default='hg',

mercurial/scmutil.py

@@ -725,9 +725,32 b' def meaningfulparents(repo, ctx):'
         return []
     return parents
 
-def getuipathfn(repo, relative=None):
-    if relative is None:
-        relative = repo.ui.configbool('ui', 'relative-paths')
+def getuipathfn(repo, legacyrelativevalue=False, forcerelativevalue=None):
+    """Return a function that produced paths for presenting to the user.
+
+    The returned function takes a repo-relative path and produces a path
+    that can be presented in the UI.
+
+    Depending on the value of ui.relative-paths, either a repo-relative or
+    cwd-relative path will be produced.
+
+    legacyrelativevalue is the value to use if ui.relative-paths=legacy
+
+    If forcerelativevalue is not None, then that value will be used regardless
+    of what ui.relative-paths is set to.
+    """
+    if forcerelativevalue is not None:
+        relative = forcerelativevalue
+    else:
+        config = repo.ui.config('ui', 'relative-paths')
+        if config == 'legacy':
+            relative = legacyrelativevalue
+        else:
+            relative = stringutil.parsebool(config)
+            if relative is None:
+                raise error.ConfigError(
+                    _("ui.relative-paths is not a boolean ('%s')") % config)
+
     if relative:
         cwd = repo.getcwd()
         pathto = repo.pathto