buildman: Try to guess the upstream commit

Buildman normally obtains the upstream commit by asking git. Provided that
the branch was created with 'git checkout -b <branch> <some_upstream>' then
this normally works.

When there is no upstream, we can try to guess one, by looking up through
the commits until we find a branch. Add a function to try this and print
a warning if buildman ends up relying on it.

Also update the documentation to match.

Signed-off-by: Simon Glass <sjg@chromium.org>
Suggested-by: Wolfgang Denk <wd@denx.de>

tools/buildman/README

@@ -310,8 +310,9 @@ branch with a valid upstream)
 $ ./tools/buildman/buildman -b <branch> -n
 
 If it can't detect the upstream branch, try checking out the branch, and
-doing something like 'git branch --set-upstream <branch> upstream/master'
-or something similar.
+doing something like 'git branch --set-upstream-to upstream/master'
+or something similar. Buildman will try to guess a suitable upstream branch
+if it can't find one (you will see a message like" Guessing upstream as ...).
 
 As an example:
 

tools/buildman/control.py

@@ -127,12 +127,12 @@ def DoBuildman(options, args, toolchains=None, make_func=None, boards=None,
         if not options.branch:
             count = 1
         else:
-            count = gitutil.CountCommitsInBranch(options.git_dir,
-                                                 options.branch)
+            count, msg = gitutil.CountCommitsInBranch(options.git_dir,
+                                                      options.branch)
             if count is None:
-                str = ("Branch '%s' not found or has no upstream" %
-                       options.branch)
-                sys.exit(col.Color(col.RED, str))
+                sys.exit(col.Color(col.RED, msg))
+            if msg:
+                print col.Color(col.YELLOW, msg)
             count += 1   # Build upstream commit also
 
     if not count: