coverity: only analyze the master branch of the main repository

We used to only execute Coverity analysis on the 'development'
branch before commit 998f0016 (Refine build limitation,
2014-01-15), which refined Coverity build limitations. While we
do not really use the 'development' branch anymore, it does
still make sense to only analyze a single branch, as otherwise
Coverity might get confused.

Re-establish the restriction such that we only analyze libgit2's
'master' branch. Also fix the message announcing why we do not
actually analyze a certain build.

script/coverity.sh

@@ -5,10 +5,10 @@ set -e
 [ -z "$COVERITY_TOKEN" ] && echo "Need to set a coverity token" && exit 1
 
 # Only run this on our branches
-echo "Pull request: $TRAVIS_PULL_REQUEST  |  Slug: $TRAVIS_REPO_SLUG"
-if [ "$TRAVIS_PULL_REQUEST" != "false" -o "$TRAVIS_REPO_SLUG" != "libgit2/libgit2" ];
+echo "Branch: $TRAVIS_BRANCH  |  Pull request: $TRAVIS_PULL_REQUEST  |  Slug: $TRAVIS_REPO_SLUG"
+if [ "$TRAVIS_BRANCH" != "master" -o "$TRAVIS_PULL_REQUEST" != "false" -o "$TRAVIS_REPO_SLUG" != "libgit2/libgit2" ];
 then
-	echo "Only analyzing 'development' on the main repo."
+	echo "Only analyzing the 'master' brach of the main repository."
 	exit 0
 fi