[tools] Create stamp file after doing things that might modify files we care about. (#3864)

We have consistency checks to verify that no unexpected files are modified
done when comparing APIs in for a pull request.

Unfortunately the check didn't take into account that checking out the
revision to do the API check against might modify some of the files in the
consistency check itself, thus triggering the consistency check.

Fix this by only verify timestamps of files modified after checkout out the
revision, which is the only thing we care about anyway.

For examples see PR #3855 or PR #3850.

tools/compare-commits.sh

@@ -160,8 +160,6 @@ for dir in Xamarin.Mac 4.5; do
 	$CP -R "$ROOT_DIR/_mac-build/Library/Frameworks/Xamarin.Mac.framework/Versions/git/lib/mono/$dir" "$OUTPUT_DIR/_mac-build/Library/Frameworks/Xamarin.Mac.framework/Versions/git/lib/mono"
 done
 
-touch "$OUTPUT_DIR/stamp"
-
 if test -z "$CURRENT_BRANCH"; then
 	echo "${BLUE}Current hash: ${WHITE}$(git log -1 --pretty="%h: %s")${BLUE} (detached)${CLEAR}"
 else
@@ -170,6 +168,11 @@ fi
 echo "${BLUE}Checking out ${WHITE}$(git log -1 --pretty="%h: %s" "$BASE_HASH")${CLEAR}...${CLEAR}"
 git checkout --quiet --force --detach "$BASE_HASH"
 
+# To ensure that our logic below doesn't modify files it shouldn't, we create a stamp
+# file, and compare the timestamps of all the files that shouldn't be modified to this
+# file's timestamp.
+touch "$OUTPUT_DIR/stamp"
+
 echo "${BLUE}Building src/...${CLEAR}"
 make -C "$ROOT_DIR/src" BUILD_DIR=../tools/comparison/build "IOS_DESTDIR=$OUTPUT_DIR/_ios-build" "MAC_DESTDIR=$OUTPUT_DIR/_mac-build" -j8