fix: ensure SemVer instance passed to inc are not modified (#427)

Author: louis-bompart

functions/inc.js

@@ -7,7 +7,10 @@ const inc = (version, release, options, identifier) => {
   }
 
   try {
-    return new SemVer(version, options).inc(release, identifier).version
+    return new SemVer(
+      version instanceof SemVer ? version.version : version,
+      options
+    ).inc(release, identifier).version
   } catch (er) {
     return null
   }

test/functions/inc.js

@@ -10,10 +10,20 @@ test('increment versions test', (t) => {
     t.equal(found, wanted, `${cmd} === ${wanted}`)
 
     const parsed = parse(pre, options)
+    const parsedAsInput = parse(pre, options)
     if (wanted) {
       parsed.inc(what, id)
       t.equal(parsed.version, wanted, `${cmd} object version updated`)
       t.equal(parsed.raw, wanted, `${cmd} object raw field updated`)
+
+      const preIncObject = JSON.stringify(parsedAsInput)
+      inc(parsedAsInput, what, options, id)
+      const postIncObject = JSON.stringify(parsedAsInput)
+      t.equal(
+        postIncObject,
+        preIncObject,
+        `${cmd} didn't modify its input`
+      )
     } else if (parsed) {
       t.throws(() => {
         parsed.inc(what, id)