tags: don't crash if unable to write tag cache

This happens with hgweb in real life, if the httpd user is unable to
write in the repository directory. Another case is doing 'hg incoming'
on a repository in the filesystem owned by someone else.

mercurial/tags.py

@@ -301,7 +301,10 b' def _readtagcache(ui, repo):'
 
 def _writetagcache(ui, repo, heads, tagfnode, cachetags):
 
+    try:
     cachefile = repo.opener('tags.cache', 'w', atomictemp=True)
+    except (OSError, IOError):
+        return
     _debug(ui, 'writing cache file %s\n' % cachefile.name)
 
     realheads = repo.heads()            # for sanity checks below

tests/test-tags

@@ -37,9 +37,17 b' hg tags'
 hg identify
 
 # repeat with cold tag cache
+echo "% identify with cold cache"
 rm -f .hg/tags.cache
 hg identify
 
+# and again, but now unable to write tag cache
+echo "% identify with unwritable cache"
+rm -f .hg/tags.cache
+chmod 555 .hg
+hg identify
+chmod 755 .hg
+
 echo "% create a branch"
 echo bb > a
 hg status

tests/test-tags.out

@@ -13,6 +13,9 b' acb14030fe0a21b60322c440ad2d20cf7685a376'
 tip                                1:b9154636be93
 first                              0:acb14030fe0a
 b9154636be93 tip
+% identify with cold cache
+b9154636be93 tip
+% identify with unwritable cache
 b9154636be93 tip
 % create a branch
 M a