cli: add --no-require-git flag

This flag prevents ripgrep from requiring one to search a git repository
in order to respect git-related ignore rules (global, .gitignore and
local excludes). This actually corresponds to behavior ripgrep had long
ago, but #934 changed that. It turns out that users were relying on this
buggy behavior. In most cases, fixing it as simple as converting one's
rules to .ignore or .rgignore files. Unfortunately, there are other use
cases---like Perforce automatically respecting .gitignore files---that
make a strong case for ripgrep to at least support this.

The UX of a flag like this is absolutely atrocious. It's so obscure that
it's really not worth explicitly calling it out anywhere. Moreover, the
error cases that occur when this flag isn't used (but its behavior is
desirable) will not be intuitive, do not seem easily detectable and will
not guide users to this flag. Nevertheless, the motivation for this is
just barely strong enough for me to begrudgingly accept this.

Fixes #1414, Closes #1416

complete/_rg

@@ -144,6 +144,8 @@ _rg() {
     + '(ignore-vcs)' # VCS ignore-file options
     "--no-ignore-vcs[don't respect version control ignore files]"
     $no'--ignore-vcs[respect version control ignore files]'
+    "--no-require-git[don't require git repository to respect gitignore rules]"
+    $no'--require-git[require git repository to respect gitignore rules]'
 
     + '(ignore-dot)' # .ignore-file options
     "--no-ignore-dot[don't respect .ignore files]"
@@ -284,6 +286,7 @@ _rg() {
     '--context-separator=[specify string used to separate non-continuous context lines in output]:separator'
     $no"--no-context-separator[don't print context separators]"
     '--debug[show debug messages]'
+    '--trace[show more verbose debug messages]'
     '--dfa-size-limit=[specify upper size limit of generated DFA]:DFA size (bytes)'
     "(1 stats)--files[show each file that would be searched (but don't search)]"
     '*--ignore-file=[specify additional ignore file]:ignore file:_files'