git-p4: create new function run_git_hook

This commit is in preparation of introducing new p4 submit hooks.

The current code in the python script git-p4.py makes the assumption
that the git hooks can be executed by subprocess.call() function.
However, when git is run on Windows, this may not work as expected.

The subprocess.call() does not cover all the use cases for properly
executing the various types of executable files on Windows.

Prepare for remediation by adding a new function, run_git_hook, that
takes 2 parameters:
* the short filename of an optionally registered git hook
* an optional list of parameters

The run_git_hook function will honor the existing behavior seen in the
current code for executing the p4-pre-submit hook:

* Hooks are looked for in core.hooksPath directory.
* If core.hooksPath is not set, then the current .git/hooks directory
  is checked.
* If the hook does not exist, the function returns True.
* If the hook file is not accessible, the function returns True.
* If the hook returns a zero exit code when executed, the function
  return True.
* If the hook returns a non-zero exit code, the function returns False.

Add the following additional functionality if git-p4.py is run on
Windows.
* If hook file is not located without an extension, search for
  any file in the associated hook directory (from the list above) that
  has the same name but with an extension.
* If the file is still not found, return True (the hook is missing)

Add a new function run_hook_command() that wraps the OS dependent
functionality for actually running the subprocess.call() with OS
dependent behavior:

If a hook file exists on Windows:
* If there is no extension, set the launch executable to be SH.EXE
  - Look for SH.EXE under the environmental variable EXEPATH in the
    bin/ directory.
  - If %EXEPATH%/bin/sh.exe exists, use this as the actual executable.
  - If %EXEPATH%/bin/sh.exe does not exist, use sh.exe
  - Execute subprocess.call() without the shell (shell=False)
* If there is an extension, execute subprocess.call() with teh shell
  (shell=True) and consider the file to be the executable.

The return value from run_hook_command() is the subprocess.call()
return value.

These functions are added in this commit, but are only staged and not
yet used.

Signed-off-by: Ben Keene <seraphire@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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