upload: Allow review.HOST.username to override email

Some users might need to use a different login name than the local
part of their email address for their Gerrit Code Review user
account.  Allow it to be overridden with the review.HOST.username
configuration variable.

Change-Id: I714469142ac7feadf09fee9c26680c0e09076b75
Signed-off-by: Shawn O. Pearce <sop@google.com>
diff --git a/subcmds/upload.py b/subcmds/upload.py
index 569e31c..b47b37b 100644
--- a/subcmds/upload.py
+++ b/subcmds/upload.py
@@ -92,6 +92,11 @@
 who you always want copied on all uploads with a non-empty --re
 argument.
 
+review.URL.username:
+
+Override the username used to connect to Gerrit Code Review.
+By default the local part of the email address is used.
+
 The URL must match the review URL listed in the manifest XML file,
 or in the .git/config within the project.  For example: