Fix `fetch_git` with `--keep` (sstephenson/ruby-build#549)

plugins/python-build/bin/python-build

@@ -394,7 +394,14 @@ fetch_git() {
       popd >&4
     fi
 
-    git clone --depth 1 --branch "$git_ref" "$git_url" "${package_name}" >&4 2>&1
+    if [ -e "${package_name}" ]; then
+      ( cd "${package_name}"
+        git fetch --depth 1 origin "+${git_ref}"
+        git checkout -q -B "$git_ref" "origin/${git_ref}"
+      ) >&4 2>&1
+    else
+      git clone --depth 1 --branch "$git_ref" "$git_url" "${package_name}" >&4 2>&1
+    fi
   else
     echo "error: please install \`git\` and try again" >&2
     exit 1

plugins/python-build/test/fetch.bats

@@ -4,6 +4,11 @@ load test_helper
 export PYTHON_BUILD_SKIP_MIRROR=1
 export PYTHON_BUILD_CACHE_PATH=
 
+setup() {
+  export PYTHON_BUILD_BUILD_PATH="${TMP}/source"
+  mkdir -p "${PYTHON_BUILD_BUILD_PATH}"
+}
+
 @test "failed download displays error message" {
   stub curl false
 
@@ -12,3 +17,36 @@ export PYTHON_BUILD_CACHE_PATH=
   assert_output_contains "> http://example.com/packages/package-1.0.0.tar.gz"
   assert_output_contains "error: failed to download package-1.0.0.tar.gz"
 }
+
+@test "fetching from git repository" {
+  stub git "clone --depth 1 --branch master http://example.com/packages/package.git package-dev : mkdir package-dev"
+
+  run_inline_definition <<DEF
+install_git "package-dev" "http://example.com/packages/package.git" master copy
+DEF
+  assert_success
+  assert_output <<OUT
+Cloning http://example.com/packages/package.git...
+Installing package-dev...
+Installed package-dev to ${TMP}/install
+OUT
+  unstub git
+}
+
+@test "updating existing git repository" {
+  mkdir -p "${PYTHON_BUILD_BUILD_PATH}/package-dev"
+  stub git \
+    "fetch --depth 1 origin +master : true" \
+    "checkout -q -B master origin/master : true"
+
+  run_inline_definition <<DEF
+install_git "package-dev" "http://example.com/packages/package.git" master copy
+DEF
+  assert_success
+  assert_output <<OUT
+Cloning http://example.com/packages/package.git...
+Installing package-dev...
+Installed package-dev to ${TMP}/install
+OUT
+  unstub git
+}