sparse: properly error out when absolute paths are used

Current logic is misleading (it says it drops only absolute paths, but
it actually drops all of them), not cross-platform (does not support Windows)
and IMO just wrong (as it should just error out if absolute paths are given).

This commit fixes it.

mercurial/sparse.py

@@ -636,10 +636,10 b' def updateconfig(repo, pats, opts, inclu'
             newexclude = set(oldexclude)
             newprofiles = set(oldprofiles)
 
-        if any(pat.startswith('/') for pat in pats):
-            repo.ui.warn(_('warning: paths cannot start with /, ignoring: %s\n')
-                         % ([pat for pat in pats if pat.startswith('/')]))
-        elif include:
+        if any(os.path.isabs(pat) for pat in pats):
+            raise error.Abort(_('paths cannot be absolute'))
+
+        if include:
             newinclude.update(pats)
         elif exclude:
             newexclude.update(pats)

tests/test-sparse.t

@@ -29,20 +29,22 b' Absolute paths outside the repo should j'
 
 #if no-windows
   $ hg debugsparse --include /foo/bar
-  warning: paths cannot start with /, ignoring: ['/foo/bar']
+  abort: paths cannot be absolute
+  [255]
   $ hg debugsparse --include '$TESTTMP/myrepo/hide'
 
   $ hg debugsparse --include '/root'
-  warning: paths cannot start with /, ignoring: ['/root']
+  abort: paths cannot be absolute
+  [255]
 #else
 TODO: See if this can be made to fail the same way as on Unix
   $ hg debugsparse --include /c/foo/bar
-  abort: c:/foo/bar not under root '$TESTTMP/myrepo' (glob)
+  abort: paths cannot be absolute
   [255]
   $ hg debugsparse --include '$TESTTMP/myrepo/hide'
 
   $ hg debugsparse --include '/c/root'
-  abort: c:/root not under root '$TESTTMP/myrepo' (glob)
+  abort: paths cannot be absolute
   [255]
 #endif