Lock the short commit IDs used to 7 characters

To prevent AppVeyor's usage of Git v2.11.0 from causing it to deploy
different Bintray versions from Travis, which still uses Git v1.8. Fixes
#744.

appveyor.yml

@@ -71,7 +71,7 @@ after_test:
   - cd %APPVEYOR_BUILD_FOLDER%
   - C:\Python27\Scripts\sphinx-build -b html docs build\docs\html
   - ps: |
-      $env:GIT_DESCRIBE = ((git describe --tags --long) | Out-String) -replace "`n|`r", ""
+      $env:GIT_DESCRIBE = ((git describe --tags --long --abbrev=7) | Out-String) -replace "`n|`r", ""
       if ($env:PLATFORM -eq 'Win32') {
         $env:PATH = "$env:PATH;C:\cygwin\bin\"
         node scripts\potomo.js

scripts/archive.js

@@ -14,6 +14,7 @@ function getGitDescription() {
     'describe',
     '--tags',
     '--long',
+    '--abbrev=7',
   ])).slice(0, -1);
   let branch = String(helpers.safeExecFileSync('git', [
     'rev-parse',