git-p4: do not terminate creating tag for unknown commit

If p4 reports a tag for a commit that git-p4 does not know
about (e.g. because it references a P4 changelist that was
imported prior to the point at which the repo was cloned into
git), make sure that the error is correctly caught and handled.
rather than just crashing.

Signed-off-by: Luke Diamand <luke@diamand.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: luked99

git-p4.py

@@ -2494,9 +2494,9 @@ def importP4Labels(self, stream, p4Labels):
                 # find the corresponding git commit; take the oldest commit
                 changelist = int(change['change'])
                 gitCommit = read_pipe(["git", "rev-list", "--max-count=1",
-                     "--reverse", ":/\[git-p4:.*change = %d\]" % changelist])
+                     "--reverse", ":/\[git-p4:.*change = %d\]" % changelist], ignore_error=True)
                 if len(gitCommit) == 0:
-                    print "could not find git commit for changelist %d" % changelist
+                    print "importing label %s: could not find git commit for changelist %d" % (name, changelist)
                 else:
                     gitCommit = gitCommit.strip()
                     commitFound = True