Make lastchange.py faster using git cat-file.

Make lastchange.py faster using git cat-file.

lastchange.py is using git show -s to get the commit message of HEAD.
It turns out that on some older versions of git that is a rather
expensive operation. Under particular circumstances it can eventually
take several minutes (see crbug.com/431463 #27).
Anyways, git cat-file is x45 faster than the equivalent show command
for the sake of printing the commit message.
$ sudo sh -c 'echo 3 > /proc/sys/vm/drop_caches'
$ time git show -s --format=%B HEAD > /dev/null
real    0m0.090s

$ sudo sh -c 'echo 3 > /proc/sys/vm/drop_caches'
$ time git cat-file commit HEAD > /dev/null
real    0m0.002s

BUG=431463
Committed: https://crrev.com/5176f672c91c461a2f87f333e2b88ab407b2bc52
Cr-Commit-Position: refs/heads/master@{#304444}

[Primiano Tucci (use gerrit), 2014-11-17 16:57:08]

primiano@chromium.org changed reviewers:
+ agable@chromium.org, thakis@chromium.org

[Nico, 2014-11-17 17:37:05]

lgtm

While reviewing, found this in git:

#define type_from_string(str) type_from_string_gently(str, -1, 0)
int type_from_string_gently(const char *str, ssize_t len, int gentle)
{
	/* early returns for success paths omitted */

	if (gentle)
		return -1;

	die("invalid object type \"%s\"", str);
}


I didn't check the `git show` history to confirm your theory though.

[Nico, 2014-11-17 17:49:14]

The CQ bit was checked by thakis@chromium.org

[commit-bot: I haz the power, 2014-11-17 17:50:02]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/726413003/1

[commit-bot: I haz the power, 2014-11-17 18:39:13]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-11-17 18:39:52]

Patchset 1 (id:??) landed as
https://crrev.com/5176f672c91c461a2f87f333e2b88ab407b2bc52
Cr-Commit-Position: refs/heads/master@{#304444}

[agable, 2014-11-17 19:30:23]

This makes a lot of sense to me. Retroactive LGTM, and thanks.