Add repo start option to create the branch based off HEAD

This makes it way easier to recover from forgetting to run repo start
before committing: just run `repo start -b new-branch`, instead of
all that tedious mucking around with reflogs.

Change-Id: I56d49dce5d027e28fbba0507ac10cd763ccfc36d
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/232712
Reviewed-by: Mike Frysinger <vapier@google.com>
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>

project.py

@@ -1697,7 +1697,7 @@ class Project(object):
 
 # Branch Management ##
 
-  def StartBranch(self, name, branch_merge=''):
+  def StartBranch(self, name, branch_merge='', revision=None):
     """Create a new branch off the manifest's revision.
     """
     if not branch_merge:
@@ -1718,7 +1718,11 @@ class Project(object):
     branch.merge = branch_merge
     if not branch.merge.startswith('refs/') and not ID_RE.match(branch_merge):
       branch.merge = R_HEADS + branch_merge
-    revid = self.GetRevisionId(all_refs)
+
+    if revision is None:
+      revid = self.GetRevisionId(all_refs)
+    else:
+      revid = self.work_git.rev_parse(revision)
 
     if head.startswith(R_HEADS):
       try:

subcmds/start.py

@@ -40,6 +40,10 @@ revision specified in the manifest.
     p.add_option('--all',
                  dest='all', action='store_true',
                  help='begin branch in all projects')
+    p.add_option('-r', '--rev', '--revision', dest='revision',
+                 help='point branch at this revision instead of upstream')
+    p.add_option('--head', dest='revision', action='store_const', const='HEAD',
+                 help='abbreviation for --rev HEAD')
 
   def ValidateOptions(self, opt, args):
     if not args:
@@ -108,7 +112,8 @@ revision specified in the manifest.
         else:
           branch_merge = self.manifest.default.revisionExpr
 
-      if not project.StartBranch(nb, branch_merge=branch_merge):
+      if not project.StartBranch(
+          nb, branch_merge=branch_merge, revision=opt.revision):
         err.append(project)
     pm.end()