lfs: default the User-Agent header for blob transfers to 'git-lfs'

The custom User-Agent for blob transfers was added in e7bb5fc4570c.  Now I've
hit another incompatibility with a server wanting the string to start with
'git' or 'git-lfs' [1].  I don't feel strongly about this either way, but a
Wireshark trace of git shows that when the Batch API is hit, the User-Agent is
'git-lfs/2.3.4'.  So this would probably ensure maximum interoperability.

This still leaves the experimental knob in, just in case.

[1] https://bitbucket.org/sdorra/scm-manager/src/095a027178888bc2b819aebfae3d2c192c858030/scm-plugins/scm-git-plugin/src/main/java/sonia/scm/web/GitUserAgentProvider.java?at=default&fileviewer=file-view-default#GitUserAgentProvider.java-117

hgext/lfs/blobstore.py

@@ -188,7 +188,7 b' class _gitlfsremote(object):'
         self.baseurl = baseurl.rstrip('/')
         useragent = repo.ui.config('experimental', 'lfs.user-agent')
         if not useragent:
-            useragent = 'mercurial/%s git/2.15.1' % util.version()
+            useragent = 'git-lfs/2.3.4 (Mercurial %s)' % util.version()
         self.urlopener = urlmod.opener(ui, authinfo, useragent)
         self.retry = ui.configint('lfs', 'retry')