Explain how to use a python3 executable not named "python3"

Signed-off-by: Elijah Newren <newren@gmail.com>

INSTALL

@@ -5,6 +5,11 @@ filter-repo only consists of a few files that need to be installed:
     This can be installed in the directory pointed to by `git --exec-path`,
     or placed anywhere in $PATH.  This is the only thing needed for basic use.
 
+    If your python3 executable is named "python" instead of "python3"
+    (this particularly appears to affect a number of Windows users),
+    then you'll also need to modify the first line of git-filter-repo
+    to replace "python3" with "python".
+
   * git_filter_repo.py
 
     If you want to make use of one of the scripts in contrib/filter-repo-demos/,
@@ -28,3 +33,7 @@ that prefer it.  However, usage of the Makefile really requires overridding
 at least a couple of the directories with sane values, e.g.
 
     $ make prefix=/usr pythondir=/usr/lib64/python3.8/site-packages install
+
+Also, the Makefile will not edit the shebang line (the first line) of
+git-filter-repo if your python executable is not named "python3";
+you'll still need to do that yourself.

README.md

@@ -46,6 +46,7 @@ installation for basic use trivial: just copy it into your $PATH.
 See [INSTALL](INSTALL) for things beyond basic usage or special cases.
 The more involved instructions are needed if you
 
+  * are working with a python3 executable named something other than "python3"
   * want to install documentation
   * want to run some of the [contrib](contrib/filter-repo-demos/) examples
   * want to create your own python filtering scripts using filter-repo as a

git-filter-repo

@@ -1584,6 +1584,8 @@ class GitUtils(object):
       contents = f.read()
     # If people replaced @@LOCALEDIR@@ string to point at their local
     # directory, undo it so we can get original source version.
+    contents = re.sub(br'^#\!/usr/bin/env python.*',
+                      br'#!/usr/bin/env python3', contents)
     contents = re.sub(br'(\("GIT_TEXTDOMAINDIR"\) or ").*"',
                       br'\1@@LOCALEDIR@@"', contents)