Setup git-svn and handle detached HEAD state in auto-rebaseline script.

Tools/Scripts/webkitpy/tool/commands/rebaseline.py

Index: Tools/Scripts/webkitpy/tool/commands/rebaseline.py
diff --git a/Tools/Scripts/webkitpy/tool/commands/rebaseline.py b/Tools/Scripts/webkitpy/tool/commands/rebaseline.py
index 9d238f4fd3dc3ddf7a1f0b28a1d2f54fd2eedd30..bb2ff336a08e989de7bd44c0a59240dded076ede 100644
--- a/Tools/Scripts/webkitpy/tool/commands/rebaseline.py
+++ b/Tools/Scripts/webkitpy/tool/commands/rebaseline.py
@@ -829,7 +829,18 @@ class AutoRebaseline(AbstractParallelRebaselineCommand):
 
         did_finish = False
         try:
-            old_branch_name = tool.scm().current_branch()

[Xianzhu, 2015/07/14 20:40:04]

What will tool.scm().current_branch() return when HEAD is detached? I think we
can just set old_branch_name to 'master' in the case because git will
automatically create 'master' when it doesn't exist.

I think we should also checkout master and set old_branch_name to master if
old_branch_name is self.AUTO_REBASELINE_BRANCH_NAME.

The right-side code also has a problem that old_branch_name has a '\n' in it and
the later checkout will crash because of it.

The following code should work:
  old_branch_name = tool.scm().current_branch()
  if not old_branch_name or old_branch_name == self.AUTO_REBASELINE_BRANCH_NAME:
    old_branch_name = 'master'
    tool.scm().checkout_branch(old_branch_name)

Perhaps we should make the above code a function because we also save/restore
current branch in RebaselineOMatic.

[joelo, 2015/07/14 23:51:01]

Is it possible to create a branch with '\n' in it's name??

[Xianzhu, 2015/07/15 15:57:12]

Just tried:
$ git checkout -tb 'a
fatal: 'a
' is not a valid branch name.

The right side code misses a strip, so will include the ending '\n' of the git
rev-parse command in old_branch_name. I think we should use scm methods if
possible.

BTW, As jsbell@ said, checking out 'master' might not be the best solution.

+            # Setup git-svn for dcommit if necessary.
+            if tool.executive.run_command(
+                    ['git', 'config', '--local', '--get-regexp', r'^svn-remote\.'],
+                    return_exit_code=True):
+                tool.executive.run_command(['git', 'auto-svn'])
+
+            # Save the current branch name and checkout a clean branch for the patch.
+            old_branch_name = tool.executive.run_command(
+                ["git", "rev-parse", "--symbolic-full-name", "HEAD"])
+            if old_branch_name == "HEAD":
+                # If HEAD is detached use commit SHA instead.
+                old_branch_name = tool.executive.run_command(["git", "rev-parse", "HEAD"])
             tool.scm().delete_branch(self.AUTO_REBASELINE_BRANCH_NAME)
             tool.scm().create_clean_branch(self.AUTO_REBASELINE_BRANCH_NAME)