branch: avoid using reserved tag names

Reported as Debian bug  #552423.

mercurial/changelog.py

@@ -218,8 +218,13 b' class changelog(revlog.revlog):'
             parseddate = "%d %d" % util.parsedate(date)
         else:
             parseddate = "%d %d" % util.makedate()
-        if extra and extra.get("branch") in ("default", ""):
+        if extra:
+            branch = extra.get("branch")
+            if branch in ("default", ""):
             del extra["branch"]
+            elif branch in (".", "null", "tip"):
+                raise error.RevlogError(_('the name \'%s\' is reserved')
+                                        % branch)
         if extra:
             extra = encodeextra(extra)
             parseddate = "%s %s" % (parseddate, extra)

mercurial/dirstate.py

@@ -202,6 +202,8 b' class dirstate(object):'
         self._pl = p1, p2
 
     def setbranch(self, branch):
+        if branch in ['tip', '.', 'null']:
+            raise util.Abort(_('the name \'%s\' is reserved') % branch)
         self._branch = branch
         self._opener("branch", "w").write(branch + '\n')
 

tests/test-branches

@@ -36,6 +36,10 b' hg add c'
 hg branch c
 hg commit -d '5 0' -m "Adding c branch"
 
+hg branch tip
+hg branch null
+hg branch .
+
 echo 'd' >d
 hg add d
 hg branch 'a branch name much longer than the default justification used by branches'

tests/test-branches.out

@@ -6,6 +6,9 b' marked working directory as branch b'
 created new head
 1 files updated, 0 files merged, 2 files removed, 0 files unresolved
 marked working directory as branch c
+abort: the name 'tip' is reserved
+abort: the name 'null' is reserved
+abort: the name '.' is reserved
 marked working directory as branch a branch name much longer than the default justification used by branches
 a branch name much longer than the default justification used by branches 7:10ff5895aa57
 b                              4:aee39cd168d0