contrib: Add script to show uncovered "new" lines

[derrickstolee]

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 in error cases do not need to be covered.

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. For example, I ran this against the 'next' branch (e82ca0) versus 'master' (f84b9b) and got the following output:

builtin/commit.c
76f2f5c1e3 builtin/commit.c 1657) write_commit_graph_reachable(get_object_directory(), 0, 0);

builtin/fsck.c
66ec0390e7 builtin/fsck.c 862) midx_argv[2] = "--object-dir";
66ec0390e7 builtin/fsck.c 863) midx_argv[3] = alt->path;
66ec0390e7 builtin/fsck.c 864) if (run_command(&midx_verify))
66ec0390e7 builtin/fsck.c 865) errors_found |= ERROR_COMMIT_GRAPH;

fsck.c
fb8952077d  214) die_errno("Could not read '%s'", path);

midx.c
56ee7ff156  949) return 0;
cc6af73c02  990) midx_report(_("failed to load pack-index for packfile %s"),
cc6af73c02  991)     e.p->pack_name);
cc6af73c02  992) break;

Commits introducing uncovered code:
Derrick Stolee      56ee7ff15: multi-pack-index: add 'verify' verb
Derrick Stolee      66ec0390e: fsck: verify multi-pack-index
Derrick Stolee      cc6af73c0: multi-pack-index: verify object offsets
Junio C Hamano      76f2f5c1e: Merge branch 'ab/commit-graph-progress' into next
René Scharfe      fb8952077: fsck: use strbuf_getline() to read skiplist file

Thanks,
-Stolee

CHANGES IN V3: I took Junio's perl script verbatim, which speeds up the performance greatly. Some of
the other sed commands needed some massaging, but also added extra cleanup. Thanks for the help!

CHANGES IN V4: I reduced the blame output using -s which decreases the width. I include a summary of the commit authors at the end to help people see the lines they wrote. This version is also copied into a build definition in the public Git project on Azure Pipelines [1]. I'll use this build definition to generate the coverage report after each "What's Cooking" email.

[1] https://git.visualstudio.com/git/_build?definitionId=5

Cc: peff@peff.net

[derrickstolee]

/submit

[gitgitgadget]

Submitted as pull.40.git.gitgitgadget@gmail.com

[derrickstolee]

/submit

[gitgitgadget]

Submitted as pull.40.v2.git.gitgitgadget@gmail.com

[derrickstolee]

/submit

[gitgitgadget]

Submitted as pull.40.v3.git.gitgitgadget@gmail.com

[derrickstolee]

/submit

[gitgitgadget]

Submitted as pull.40.v4.git.gitgitgadget@gmail.com