On windows Mercurial installed with easy_install typically creates
an hg.bat batch file in Python Scripts directory, which cannot be used
with CreateProcess unless full path is specified. Work around by
launching hg via cmd.exe /c.

Additionally, fix a rare FormatMessageW crash.

Fixes #3093.

R=golang-dev, rsc, alex.brainman, aram, jdpoirier, mattn.jp
CC=golang-dev
https://golang.org/cl/5937043