fix: normalize git show signature option to false

This disables `git log` showing signatures (`--no-show-signature`)
unless `showSignature` is set to a truthy value.  This way differences
in developers` local configuration (`log.showSignature`) do not affect
tooling operation.  Callers of `gitRawCommits` will need to explicitly
request signatures (via `rawGitOpts`) if they are desired, instead of
the current behavior of the presence of signatures being undefined.

Fixes conventional-changelog/commitlint#2118

Author: thesmiley1

packages/git-raw-commits/index.js

@@ -19,6 +19,7 @@ function normalizeGitOpts (gitOpts) {
   gitOpts = gitOpts || {}
   gitOpts.format = gitOpts.format || '%B'
   gitOpts.from = gitOpts.from || ''
+  gitOpts.showSignature = gitOpts.showSignature || false
   gitOpts.to = gitOpts.to || 'HEAD'
   return gitOpts
 }