lfs: add an experimental config to override User-Agent for the blob transfer

This will allow developers to test against various server implementations.  I
didn't put it under [devel] because it's possible that some user needs to use it
in the field.

hgext/lfs/__init__.py

@@ -64,6 +64,10 b" testedwith = 'ships-with-hg-core'"
 configtable = {}
 configitem = registrar.configitem(configtable)
 
+configitem('experimental', 'lfs.user-agent',
+    default=None,
+)
+
 configitem('lfs', 'url',
     default=configitem.dynamicdefault,
 )

hgext/lfs/blobstore.py

@@ -125,6 +125,8 b' class _gitlfsremote(object):'
         self.ui = ui
         baseurl, authinfo = url.authinfo()
         self.baseurl = baseurl.rstrip('/')
+        useragent = repo.ui.config('experimental', 'lfs.user-agent')
+        if not useragent:
         useragent = 'mercurial/%s git/2.15.1' % util.version()
         self.urlopener = urlmod.opener(ui, authinfo, useragent)
         self.retry = ui.configint('lfs', 'retry')