remotefilelog: acquire lock before writing requirements on clone

Performing a shallow clone in remotefilelog does not acquire lock. This leads to
following warning when we try to write some requirements in store:

```
--- /home/gps/src/hg-committed/tests/test-remotefilelog-share.t
+++ /home/gps/src/hg-committed/tests/test-remotefilelog-share.t#safe.err
@@ -28,6 +28,7 @@


   $ hgcloneshallow ssh://user@dummy/master source --noupdate -q
+  devel-warn: write with no lock: "requires" at: /home/gps/src/hg-committed/mercurial/scmutil.py:1505 (writerequires)
   $ hg share source dest
   updating working directory
   1 files updated, 0 files merged, 0 files removed, 0 files unresolved
```

Let's lock before writing the requirements file.

Another solution which I can think of is not warn about missing lock when
writing to requires file in store.

Differential Revision: https://phab.mercurial-scm.org/D8952

hgext/remotefilelog/__init__.py

@@ -362,7 +362,10 b' def cloneshallow(orig, ui, repo, *args, '
                         self.unfiltered().__class__,
                     )
                 self.requirements.add(constants.SHALLOWREPO_REQUIREMENT)
-                scmutil.writereporequirements(self)
+                with self.lock():
+                    # acquire store lock before writing requirements as some
+                    # requirements might be written to .hg/store/requires
+                    scmutil.writereporequirements(self)
 
                 # Since setupclient hadn't been called, exchange.pull was not
                 # wrapped. So we need to manually invoke our version of it.