Prevent false positives when checking for changes in src/doc.

When changing cargo versions from stable to nightly, it's possible
that the Cargo.lock format changes ever so slightly. This leaves
the `Cargo.lock` file changed which is picked up by `git status`.

This patch adjusts the `git status` invocation to limit itself
to the CWD.

ci/validate-man.sh

@@ -5,7 +5,7 @@ set -e
 
 cd src/doc
 
-changes=$(git status --porcelain)
+changes=$(git status --porcelain -- .)
 if [ -n "$changes" ]
 then
     echo "git directory must be clean before running this script."
@@ -14,10 +14,10 @@ fi
 
 ./build-man.sh
 
-changes=$(git status --porcelain)
+changes=$(git status --porcelain -- .)
 if [ -n "$changes" ]
 then
-    echo "Detected changes in man pages:"
+    echo "Detected changes of man pages in src/doc:"
     echo "$changes"
     echo
     echo "Please run './build-man.sh' in the src/doc directory to rebuild the"