hghave: add "absimport" feature to check "absolute_import" in __future__

This patch adds "absimport" feature to check whether "absolute_import"
exists in __future__, which means supporting module loading by
absolute name.

This check is needed for portability of test code using
"absolute_import", because Python earlier than 2.5 doesn't support it.

tests/hghave.py

@@ -277,6 +277,11 b' def has_msys():'
 def has_aix():
     return sys.platform.startswith("aix")
 
+def has_absimport():
+    import __future__
+    from mercurial import util
+    return util.safehasattr(__future__, "absolute_import")
+
 checks = {
     "true": (lambda: True, "yak shaving"),
     "false": (lambda: False, "nail clipper"),
@@ -318,4 +323,5 b' checks = {'
     "windows": (has_windows, "Windows"),
     "msys": (has_msys, "Windows with MSYS"),
     "aix": (has_aix, "AIX"),
+    "absimport": (has_absimport, "absolute_import in __future__"),
 }