tests: use jshint when available to check .js files

tests/test-check-jshint.t

@@ -0,0 +1,12 b''
+#require test-repo jshint hg10
+
+  $ . "$TESTDIR/helpers-testrepo.sh"
+
+run jshint on all tracked files ending in .js except vendored dependencies
+
+  $ cd "`dirname "$TESTDIR"`"
+
+  $ testrepohg locate 'set:**.js' \
+  > -X mercurial/templates/static/excanvas.js \
+  > 2>/dev/null \
+  > | xargs jshint

tests/hghave.py

@@ -444,6 +444,10 b' def has_clang_format():'
     return matchoutput("clang-format --help",
                        br"^OVERVIEW: A tool to format C/C\+\+[^ ]+ code.")
 
+@check("jshint", "JSHint static code analysis tool")
+def has_jshint():
+    return matchoutput("jshint --version 2>&1", br"jshint v")
+
 @check("pygments", "Pygments source highlighting library")
 def has_pygments():
     try: