[git-llvm] Do not reinvent `@{upstream}` (take 2)

[davezarzycki]

Summary

This makes git-llvm more of a thin wrapper around git while temporarily maintaining backwards compatibility with past git-llvm behavior.

Using @{upstream} makes git-llvm more robust when used with a nontrivial local repository.

[mehdi_amini]

Works for me, but let's confirm with @thakis

[davezarzycki]

In D67389#1665663, @mehdi_amini wrote:

Works for me, but let's confirm with @thakis

Hi @mehdi_amini,

In the comments of D67262, @thakis said: "if there's a warning however, it'd be nice if it included git checkout -b myNewBranch origin/master as suggested command to run, since I found that difficult to find. (Thanks for mentioning it!)"

[thakis]

Looks great!

[davezarzycki]

r372070

llvm/utils/git-svn/git-llvm

if sys.platform == 'win32' and not cmd.endswith('.exe'):
         cmd += '.exe'
     for path in os.environ["PATH"].split(os.pathsep):
         if os.access(os.path.join(path, cmd), os.X_OK):
             return True
     return False
 
 
 def get_default_rev_range():
-    # Get the branch tracked by the current branch, as set by
-    # git branch --set-upstream-to  See http://serverfault.com/a/352236/38694.
-    cur_branch = git('rev-parse', '--symbolic-full-name', 'HEAD')
-    upstream_branch = git('for-each-ref', '--format=%(upstream:short)',
-                          cur_branch)
-    if not upstream_branch:
-        upstream_branch = 'origin/master'
-
     # Get the newest common ancestor between HEAD and our upstream branch.
-    upstream_rev = git('merge-base', 'HEAD', upstream_branch)
+    upstream_rev = git('merge-base', 'HEAD', '@{upstream}', ignore_errors=True)
+    if not upstream_rev:
+        eprint("Warning: git-llvm assumes that origin/master is the upstream "
+               "branch but git does not.")
+        eprint("To make this warning go away: git branch -u origin/master")
+        eprint("To avoid this warning when creating branches: "
+               "git checkout -b MyBranchName origin/master")
+        upstream_rev = git('merge-base', 'HEAD', 'origin/master')
+
     return '%s..' % upstream_rev
 
 
 def get_revs_to_push(rev_range):
     if not rev_range:
         rev_range = get_default_rev_range()
     # Use git show rather than some plumbing command to figure out which revs
     # are in rev_range because it handles single revs (HEAD^) and ranges