destutil: use cached branch information instead of query for efficiency

Before this patch, calculation of "the tipmost branch head on current
branch" uses revset query "max(.::(head() and branch(BRANCH)))", but
this isn't efficiency, because:

  - head() predicate lists up heads on all branches, but
  - branch() predicate eliminates heads on other branches

In addition to it, without "literal:" prefix for branch name,
branch(BRANCH) tries to (1) look up BRANCH in "repo.branchmap()" and
(2) look up BRANCH as symbol name again, if there is no branch
matching against BRANCH. The latter looking up is obviously redundant.

This patch uses repo.branchheads(closed=True) to get all branch heads
on specified branch instead of "head() and branch(BRANCH)" revset
query part.

This patch also makes catching RepoLookupError meaningless, because it
is only raised by revset predicate "branch()". But "currentbranch in
repo.branchmap()" can detect whether currentbranch actually exists or
not.

Therefore, this patch replaces try/except for RepoLookupError by
if/else for "currentbranch in repo.branchmap()".

mercurial/destutil.py

@@ -92,12 +92,13 @@ def _destupdatebranch(repo, clean, check):
     wc = repo[None]
     movemark = node = None
     currentbranch = wc.branch()
-    try:
-        node = repo.revs('max(.::(head() and branch(%s)))'
-                         , currentbranch).first()
+    if currentbranch in repo.branchmap():
+        heads = repo.branchheads(currentbranch, closed=True)
+        if heads:
+            node = repo.revs('max(.::(%ln))', heads).first()
         if bookmarks.isactivewdirparent(repo):
             movemark = repo['.'].node()
-    except error.RepoLookupError:
+    else:
         if currentbranch == 'default': # no default branch!
             node = repo.lookup('tip') # update to tip
         else: