hgweb: don't call sys.exit() in httpservice.run()
If I'm reading the code correctly, `mercurial.server.createservice()`
can return an hgweb service or one of three types of command server
services. The caller then calls `mercurial.server.runservice()`,
passing it the returned service's run method. Only the hgweb service
was calling `sys.exit()`. It has been that way since
8d44649df03b
(refactor ssh server., 2006-06-04). That commit message doesn't
provide any explanation. Let's clean up and have the code follow the
usual return path into the `dispatch` module.
After this patch, there should be no remaining places left where we
call `sys.exit()` except for valid uses in the `dispatch` and `worker`
modules.
Differential Revision: https://phab.mercurial-scm.org/D9272
$ hg init
$ cat > a <<EOF
> a
> b
> c
> EOF
$ hg ci -Am adda
adding a
$ cat > a <<EOF
> d
> e
> f
> EOF
$ hg ci -m moda
$ hg diff --reverse -r0 -r1
diff -r 2855cdcfcbb7 -r 8e1805a3cf6e a
--- a/a Thu Jan 01 00:00:00 1970 +0000
+++ b/a Thu Jan 01 00:00:00 1970 +0000
@@ -1,3 +1,3 @@
-d
-e
-f
+a
+b
+c
$ cat >> a <<EOF
> g
> h
> EOF
$ hg diff --reverse --nodates
diff -r 2855cdcfcbb7 a
--- a/a
+++ b/a
@@ -1,5 +1,3 @@
d
e
f
-g
-h
should show removed file 'a' as being added
$ hg revert a
$ hg rm a
$ hg diff --reverse --nodates a
diff -r 2855cdcfcbb7 a
--- /dev/null
+++ b/a
@@ -0,0 +1,3 @@
+d
+e
+f
should show added file 'b' as being removed
$ echo b >> b
$ hg add b
$ hg diff --reverse --nodates b
diff -r 2855cdcfcbb7 b
--- a/b
+++ /dev/null
@@ -1,1 +0,0 @@
-b