Miscellaneous improvements

I made a bunch of changes in the source:

- Directly insert the output of `git show` into the target buffer, instead of obtaining the output as string and inserting it. 
- Directly call "git" instead of going through the shell.  Notably, use `process-lines` and `call-process` instead of `shell-command-to-string`.
- Use real file name functions to manipulate file names.  Notably, use `file-relative-name` instead of `s-chop-prefix` to obtain a relative file name, and `file-name-as-directory` instead of `concat` to append a trailing slash.  As a side-effect, the dependency on s.el became redundant and is removed.
- Follow the recommendation in the docstring of `buffer-file-name` to use absolute file names, and put an absolute file name into `buffer-file-name` of the git timemachine buffer.

While making these changes, I noticed that the indentation of the source code is rather odd.  It seems to exlusively use a single space, which for some macros is quite different from the standard indentation.  Is there a reason for this?