t2300: "git --exec-path" is not usable in $PATH on Windows as-is

The "git" command prepends the exec-path to the PATH environment variable for processes it spawns. That is how ". git-sh-setup" in our scripted Porcelains can find the dot-sourced file in the exec-path location that is not usually on user's PATH. When t2300 runs, because it is not spawned by the "git" command, the scriptlet being tested did not run with a realistic setting of PATH environment. It lacked the exec-path on the PATH, and failed to find the dot-sourced file. A recent update to t2300 attempted to fix this, with "PATH=$(git --exec-path):$PATH", which has been the recommended way around v1.6.0 days (a script whose original was written before that release that survives to this day is likely to have such a line). However, the "git --exec-path" command outputs C:\path\to\exec\dir (not /c/path/to/exec/dir) on Windows; the recent update failed to consider the problem that comes from it. Even though Git itself, when doing the equivalent internally, does so in a platform native way (i.e. on Windows, C:\path\to\exec\dir is prepended to the existing value of %PATH% using ';' as a component separator), the result is further massaged by bash and gets turned into $PATH that uses /c/path/to/exec/dir with ':' separating the components, which is the form understood by bash, so scripted Porcelains find commands from PATH correctly. An end user script written in shell, however, cannot prepend "C:\path\to\exec\dir:" to the existing value of $PATH and expect bash to magically turn it into the form it understands. In other words, "PATH=$(git --exec-path):$PATH" does not work as an emulation of what "Git" internally does to the PATH on Windows. To correctly emulate how exec-path is prepended to the PATH environment internally on Windows, we'd need to convert C:\git-sdk-64\usr\src\git to at least /c\git-sdk-64\usr\src\git ourselves before prepending it to PATH. Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>