CLI/git: add @timed decorator, time a few functions

Perf will show which binaries are using the CPU cycles, and standard
python profilers will show which python functions, but neither is
great at finding which call to an external binary is taking time, or
locating I/O hotspots.

notmuch-git.py

@@ -89,6 +89,20 @@ def _xapian_unquote(string):
     return string
 
 
+def timed(fn):
+    """Timer decorator"""
+    from time import perf_counter
+
+    def inner(*args, **kwargs):
+        start_time = perf_counter()
+        rval = fn(*args, **kwargs)
+        end_time = perf_counter()
+        _LOG.info('{0}: {1:.8f}s elapsed'.format(fn.__name__, end_time - start_time))
+        return rval
+
+    return inner
+
+
 class SubprocessError(RuntimeError):
     "A subprocess exited with a nonzero status"
     def __init__(self, args, status, stdout=None, stderr=None):
@@ -321,6 +335,7 @@ def commit(treeish='HEAD', message=None):
         _git(args=['read-tree', treeish], wait=True)
         raise
 
+@timed
 def _update_index(status):
     with _git(
             args=['update-index', '--index-info'],
@@ -561,6 +576,7 @@ def _is_unmerged(ref='@{upstream}'):
     return base != fetch_head
 
 
+@timed
 def get_status():
     status = {
         'deleted': {},
@@ -581,7 +597,7 @@ def get_status():
     _os.remove(index)
     return status
 
-
+@timed
 def _index_tags():
     "Write notmuch tags to the nmbug.index."
     path = _os.path.join(NOTMUCH_GIT_DIR, 'nmbug.index')
@@ -630,6 +646,7 @@ def _index_tags_for_message(id, status, tags):
         yield '{mode} {hash}\t{path}\n'.format(mode=mode, hash=hash, path=path)
 
 
+@timed
 def _diff_index(index, filter):
     """
     Get an {id: {tag, ...}} dict for a given filter.