fix: Lazily import git-rev-sync in non-production

[Rob--W]

A simple and effective way to fix #1916.

An equally valid way to fix this issue, without any runtime dependencies, is to inject the version with DefinePlugin in webpack.config.js, but refactoring the code and tests to fix that would take more effort (and ultimately, still need git-rev-sync or a replacement to determine the version), so I opted for the simplest way to fix this.

[coveralls]

Coverage remained the same at 100.0% when pulling 264a328 on Rob--W:issue-1916-git-rev-sync-logspam into fc91c36 on mozilla:master.

[rpl]

Thanks for taking a look into how to fix this issue, the proposed fix looks reasonable to me but there is an additional small changes that I think we may want to apply in this PR:

I suspected that, with the current version of this PR, webpack is going to bundle git-rev-sync into the generated dist/web-ext.js and I confirmed locally that it is actually the case.

To avoid that we have to change the webpack.config.js file to add "devDependencies" to the excludeFromBundle array.

As an additional side note, I was also surprised that npm ci didn't complain in the travis job because the package-lock.json has not been updated based on the new package.json, but I guess that it may be because by moving the existing dependency from "dependencies" to "devDependencies" does only change some of the deps listed in the package-lock.json to be marked as dev: true, this is the diff that I see locally if I explicitly refresh the package-lock.json:

diff --git a/package-lock.json b/package-lock.json
index e00ec7f..ade9365 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -8028,6 +8028,7 @@
       "version": "2.0.0",
       "resolved": "https://registry.npmjs.org/git-rev-sync/-/git-rev-sync-2.0.0.tgz",
       "integrity": "sha512-vnHFv2eocTmt/wHqZm3ksxtVshK4vptT0cEoumk6hAYRFx3do6Qo7xHBTBCv29+r3ZZCQOQ1i328MUCsYF7AUw==",
+      "dev": true,
       "requires": {
         "escape-string-regexp": "1.0.5",
         "graceful-fs": "4.1.15",
@@ -8037,12 +8038,14 @@
         "graceful-fs": {
           "version": "4.1.15",
           "resolved": "https://registry.npmjs.org/graceful-fs/-/graceful-fs-4.1.15.tgz",
-          "integrity": "sha512-6uHUhOPEBgQ24HM+r6b/QwWfZq+yiFcipKFrOFiBEnWdy5sdzYoi+pJeQaPI5qOLRFqWmAXUPQNsielzdLoecA=="
+          "integrity": "sha512-6uHUhOPEBgQ24HM+r6b/QwWfZq+yiFcipKFrOFiBEnWdy5sdzYoi+pJeQaPI5qOLRFqWmAXUPQNsielzdLoecA==",
+          "dev": true
         },
         "shelljs": {
           "version": "0.7.7",
           "resolved": "https://registry.npmjs.org/shelljs/-/shelljs-0.7.7.tgz",
           "integrity": "sha1-svXHfvlxSPS09uImguELuoZnz/E=",
+          "dev": true,
           "requires": {
             "glob": "^7.0.0",
             "interpret": "^1.0.0",
@@ -8370,7 +8373,8 @@
     "interpret": {
       "version": "1.2.0",
       "resolved": "https://registry.npmjs.org/interpret/-/interpret-1.2.0.tgz",
-      "integrity": "sha512-mT34yGKMNceBQUoVn7iCDKDntA7SC6gycMAWzGx1z/CMCTV7b2AAtXlo3nRyHZ1FelRkQbQjprHSYGwzLtkVbw=="
+      "integrity": "sha512-mT34yGKMNceBQUoVn7iCDKDntA7SC6gycMAWzGx1z/CMCTV7b2AAtXlo3nRyHZ1FelRkQbQjprHSYGwzLtkVbw==",
+      "dev": true
     },
     "invariant": {
       "version": "2.2.4",
@@ -11096,7 +11100,8 @@
     "path-parse": {
       "version": "1.0.6",
       "resolved": "https://registry.npmjs.org/path-parse/-/path-parse-1.0.6.tgz",
-      "integrity": "sha512-GSmOT2EbHrINBf9SR7CDELwlJ8AENk3Qn7OikK4nFYAu3Ote2+JYNVvkpAEQm3/TLNEJFD/xZJjzyxg3KBWOzw=="
+      "integrity": "sha512-GSmOT2EbHrINBf9SR7CDELwlJ8AENk3Qn7OikK4nFYAu3Ote2+JYNVvkpAEQm3/TLNEJFD/xZJjzyxg3KBWOzw==",
+      "dev": true
     },
     "path-to-regexp": {
       "version": "1.8.0",
@@ -11583,6 +11588,7 @@
       "version": "0.6.2",
       "resolved": "https://registry.npmjs.org/rechoir/-/rechoir-0.6.2.tgz",
       "integrity": "sha1-hSBLVNuoLVdC4oyWdW70OvUOM4Q=",
+      "dev": true,
       "requires": {
         "resolve": "^1.1.6"
       }
@@ -11861,6 +11867,7 @@
       "version": "1.15.1",
       "resolved": "https://registry.npmjs.org/resolve/-/resolve-1.15.1.tgz",
       "integrity": "sha512-84oo6ZTtoTUpjgNEr5SJyzQhzL72gaRodsSfyxC/AXRvwu0Yse9H8eF9IpGo7b8YetZhlI6v7ZQ6bKBFV/6S7w==",
+      "dev": true,
       "requires": {
         "path-parse": "^1.0.6"
       }

We may actually apply this changes in this PR, but I don't think is a big deal even if we don't (at some point renovatebot may likely do it for us when it creates the next PR for a new dependency update).