add -F/--fixed-strings as alias for -Q/--literal

-F is compatible with POSIX grep(1)
http://pubs.opengroup.org/onlinepubs/9699919799/utilities/grep.html

--fixed-strings is compatible with GNU grep(1)
https://www.gnu.org/software/grep/manual/grep.html#grep-Programs

This is the most annoying (to me) muscle-memory difference from grep.
document -F/--fixed-strings
add bash completion for -F/--fixed-strings
Improve logging for skipping ignore file and Git excludesfile
Use `--path` with `git config` to get the resolved excludesfile

Fixes https://github.com/ggreer/the_silver_searcher/issues/159
Allow unlimited depth.

Add option --depth=-1 for unlimited search depth.
Add debug log for files loaded in `load_ignore_patterns`
Fix arguments to `log_debug` in `path_ignore_search`
src/lang.c: Add Mercury language (*.m, *.moo)

Mercury language (http://www.mercurylang.org)
tests/list_file_types.t: Add test for Mercury file types
tests/list_file_types.t: add missing whitespace
Makefile.w32: using $(GREP) inside VERSION, also evaluate the version only once
Merge pull request #548 from blueyed/debug-fix-args

Fix arguments to `log_debug` in `path_ignore_search`
Merge branch 'dash-F' of git://github.com/jpouellet/the_silver_searcher into jpouellet-dash-F

Conflicts:
	src/options.c
Merge branch 'jpouellet-dash-F'

Conflicts:
	src/options.c
Merge pull request #539 from kevinxucs/max-depth

Allow unlimited depth.
Merge pull request #537 from blueyed/fix-git-excludesfile-expand

Expand core.excludesfile setting from "git config"
Update README.md

Update Ack link to the now maintained ack2
Change constant name from MAX_PATH to _MAX_PATH so the code could be compiled
Merge pull request #562 from poxu/max_path_variable_typo

Change constant name from MAX_PATH to _MAX_PATH so the code could be com...