In a shell snippet meant to be sourced by other shell scripts, an
opening #! line does more harm than good.

The harm:

 - When the shell library is sourced, the interpreter and options from
   the #! line are not used.  Specifying a particular shell can
   confuse the reader into thinking it is safe for the shell library
   to rely on idiosyncrasies of that shell.

 - Using #! instead of a plain comment drops a helpful visual clue
   that this is a shell library and not a self-contained script.

 - Tools such as lintian can use a #! line to tell when an
   installation script has failed by forgetting to set a script
   executable.  This check does not work if shell libraries also start
   with a #! line.

The good:

 - Text editors notice the #! line and use it for syntax highlighting
   if you try to edit the installed scripts (without ".sh" suffix) in
   place.

The use of the #! for file type detection is not needed because Git's
shell libraries are meant...