Remove "inc" directory from .gitignore

There is no good reason to ignore the "inc" directory in git. Its
contents are derived, but from external sources, and checked in. Also,
they are not normally regenerated unless Makefile.PL is executed.
MBS-7497: Update bundled Module::Install modules

The Module::Install modules included in the distribution ("inc"
directory) were out of date (last updated in March 2010, as part of
commit ab86c78a0f4206580ce61fb8d9d36fbf1b49708c). In particular, this
meant that File::Copy::Recursive was missing. It is used by
Module::Install:Catalyst, which had a bug at the time causing its
dependency not to be included in the "inc" bundle:
https://rt.cpan.org/Public/Bug/Display.html?id=57019
More improvements to INSTALL.md
MBS-6682: Mention European FTP mirror in INSTALL.md
Fix warning in InitDb.pl

Comparing a usually undefined value to the empty string caused a
warning that was harmless, but caused confusion for people setting up
a MusicBrainz mirror server.
Typographical fixes to README.md
Merge remote-tracking branch 'chirlu/install_hints' into beta

* chirlu/install_hints:
  Typographical fixes to README.md
  Fix warning in InitDb.pl
  MBS-6682: Mention European FTP mirror in INSTALL.md
  More improvements to INSTALL.md
  MBS-7497: Update bundled Module::Install modules
  Remove "inc" directory from .gitignore