contrib: add coverage-diff script

We have coverage targets in our Makefile for using gcov to display line
coverage based on our test suite. The way I like to do it is to run:

    make coverage-test
    make coverage-report

This leaves the repo in a state where every X.c file that was covered has
an X.c.gcov file containing the coverage counts for every line, and "#####"
at every uncovered line.

There have been a few bugs in recent patches what would have been caught
if the test suite covered those blocks (including a few of mine). I want
to work towards a "sensible" amount of coverage on new topics. In my opinion,
this means that any logic should be covered, but the 'die()' blocks covering
very unlikely (or near-impossible) situations may not warrant coverage.

It is important to not measure the coverage of the codebase by what old code
is not covered. To help, I created the 'contrib/coverage-diff.sh' script.
After creating the coverage statistics at a version (say, 'topic') you can
then run

    contrib/coverage-diff.sh base topic

to see the lines added between 'base' and 'topic' that are not covered by the
test suite. The output uses 'git blame -s' format so you can find the commits
responsible and view the line numbers for quick access to the context, but
trims leading tabs in the file contents to reduce output width.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/coverage-diff.sh

@@ -0,0 +1,108 @@
+#!/bin/sh
+
+# Usage: Run 'contrib/coverage-diff.sh <version1> <version2>' from source-root
+# after running
+#
+#     make coverage-test
+#     make coverage-report
+#
+# while checked out at <version2>. This script combines the *.gcov files
+# generated by the 'make' commands above with 'git diff <version1> <version2>'
+# to report new lines that are not covered by the test suite.
+
+V1=$1
+V2=$2
+
+diff_lines () {
+	perl -e '
+		my $line_num;
+		while (<>) {
+			# Hunk header?  Grab the beginning in postimage.
+			if (/^@@ -\d+(?:,\d+)? \+(\d+)(?:,\d+)? @@/) {
+				$line_num = $1;
+				next;
+			}
+
+			# Have we seen a hunk?  Ignore "diff --git" etc.
+			next unless defined $line_num;
+
+			# Deleted line? Ignore.
+			if (/^-/) {
+				next;
+			}
+
+			# Show only the line number of added lines.
+			if (/^\+/) {
+				print "$line_num\n";
+			}
+			# Either common context or added line appear in
+			# the postimage.  Count it.
+			$line_num++;
+		}
+	'
+}
+
+files=$(git diff --name-only "$V1" "$V2" -- \*.c)
+
+# create empty file
+>coverage-data.txt
+
+for file in $files
+do
+	git diff "$V1" "$V2" -- "$file" |
+	diff_lines |
+	sort >new_lines.txt
+
+	if ! test -s new_lines.txt
+	then
+		continue
+	fi
+
+	hash_file=$(echo $file | sed "s/\//\#/")
+
+	if ! test -s "$hash_file.gcov"
+	then
+		continue
+	fi
+
+	sed -ne '/#####:/{
+			s/    #####://
+			s/:.*//
+			s/ //g
+			p
+		}' "$hash_file.gcov" |
+	sort >uncovered_lines.txt
+
+	comm -12 uncovered_lines.txt new_lines.txt |
+	sed -e 's/$/\)/' |
+	sed -e 's/^/ /' >uncovered_new_lines.txt
+
+	grep -q '[^[:space:]]' <uncovered_new_lines.txt &&
+	echo $file >>coverage-data.txt &&
+	git blame -s "$V2" -- "$file" |
+	sed 's/\t//g' |
+	grep -f uncovered_new_lines.txt >>coverage-data.txt &&
+	echo >>coverage-data.txt
+
+	rm -f new_lines.txt uncovered_lines.txt uncovered_new_lines.txt
+done
+
+cat coverage-data.txt
+
+echo "Commits introducing uncovered code:"
+
+commit_list=$(cat coverage-data.txt |
+	grep -E '^[0-9a-f]{7,} ' |
+	awk '{print $1;}' |
+	sort |
+	uniq)
+
+(
+	for commit in $commit_list
+	do
+		git log --no-decorate --pretty=format:'%an      %h: %s' -1 $commit
+		echo
+	done
+) | sort
+
+rm coverage-data.txt