 
 (defun git-get-string-sha1 (string)
   "Read a SHA1 from the specified string."
-  (let ((pos (string-match "[0-9a-f]\\{40\\}" string)))
-    (and pos (substring string pos (match-end 0)))))
+  (and string
+       (string-match "[0-9a-f]\\{40\\}" string)
+       (match-string 0 string)))
 
 (defun git-get-committer-name ()
   "Return the name to use as GIT_COMMITTER_NAME."

(defun git-write-tree (&optional index-file)
  "Call git-write-tree and return the resulting tree SHA1 as a string."
  (git-get-string-sha1
   (with-output-to-string
     (with-current-buffer standard-output
       (git-call-process-env t
        (if index-file `(("GIT_INDEX_FILE" . ,index-file)) nil)
        "write-tree")))))

(defun git-commit-tree (buffer tree head)
  "Call git-commit-tree with buffer as input and return the resulting commit SHA1."

             (append (git-get-env-strings env) (list "git") args))
    (apply #'call-process "git" nil buffer nil args)))

(defun git-call-process-env-string (env &rest args)
  "Wrapper for call-process that sets environment strings, and returns the process output as a string."
  (with-temp-buffer
    (and (eq 0 (apply #' git-call-process-env t env args))
         (buffer-string))))

(defun git-run-process-region (buffer start end program args)
  "Run a git process with a buffer region as input."
  (let ((output-buffer (current-buffer))
