[tools] Add --abbrev-ref to get the right branch name of HEAD

Add --abbrev-ref to get the right branch name of HEAD rather than checksum to return back to original branch.

Without it will make merge tool to go to unnamed branch.

Somehow old PR apache#548 could not be reopen so submit new one. Sorry

Author: Henry Saputra <[email protected]>

Closes apache#552 from hsaputra/fix_merge_tools_to_get_original_head and squashes the following commits:

98d2d97 [Henry Saputra] [tools] Add --abbrev-ref to get the right branch name of HEAD rather than checksum to return back to original branch.

tools/merge_flink_pr.py

@@ -92,7 +92,7 @@ def continue_maybe(prompt):
         fail("Okay, exiting")
 
 
-original_head = run_cmd("git rev-parse HEAD")[:8]
+original_head = run_cmd("git rev-parse --abbrev-ref HEAD").rstrip("/\n")
 
 
 def clean_up():