remember_version() only writes version if called in a Mercurial repository.

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remember_version() only writes version if called in a Mercurial repository.
forget_version() resets version only if remember_version() wrote it.

manifest hash: b30df9d93c233f4bf07150cc5067f294a98c16f4
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mercurial/version.py

@@ -15,6 +15,7 b' import re'
 import time
 
 unknown_version = 'unknown'
+remembered_version = False
 
 def get_version():
     """Return version information if available."""
@@ -34,6 +35,8 b' def write_version(version):'
 
 def remember_version():
     """Store version information."""
+    global remembered_version
+    if os.access(".hg", os.F_OK):
     f = os.popen("hg identify 2>/dev/null")  # use real hg installation
     ident = f.read()[:-1]
     if not f.close() and ident:
@@ -52,11 +55,11 b' def remember_version():'
                     break
         if modified:
             version += time.strftime('+%Y%m%d')
-    else:
-        version = unknown_version
+            remembered_version = True
     write_version(version)
 
 def forget_version():
     """Remove version information."""
+    if remembered_version:
     write_version(unknown_version)