integrations/perforce: Display changelist ID as a p4web link.

The general regex-based linkify in zulip does not allow '@' as the
prefix. In any case we know here that it is definitely a changelist
number, which is better.

Adding P4_WEB="https://p4web.example.com" to the config will enable
this behaviour. If P4_WEB is absent from the config, or has the
value None, no link is inserted.

integrations/perforce/zulip_change-commit.py

@@ -76,6 +76,15 @@ if destination is None:
     # Don't forward the notice anywhere
     sys.exit(0)
 
+change = metadata["change"]
+p4web = None
+if hasattr(config, "P4_WEB"):
+    p4web = config.P4_WEB
+
+if p4web is not None:
+    # linkify the change number
+    change = '[{change}]({p4web}/{change}?ac=10)'.format(p4web=p4web, change=change)
+
 message = """**{user}** committed revision @{change} to `{path}`.
 
 ```quote
@@ -83,7 +92,7 @@ message = """**{user}** committed revision @{change} to `{path}`.
 ```
 """.format(
     user=metadata["user"],
-    change=metadata["change"],
+    change=change,
     path=changeroot,
     desc=metadata["desc"])  # type: str
 

integrations/perforce/zulip_perforce_config.py

@@ -26,6 +26,10 @@ ZULIP_USER = "p4-bot@example.com"
 ZULIP_API_KEY = "0123456789abcdef0123456789abcdef"
 ZULIP_SITE = "https://zulip.example.com"
 
+# Set this to point at a p4web installation to get changelist IDs as links
+# P4_WEB = "https://p4web.example.com"
+P4_WEB = None
+
 # commit_notice_destination() lets you customize where commit notices
 # are sent to with the full power of a Python function.
 #