meson: prefer shell at "/bin/sh"

meson: prefer shell at "/bin/sh"

Meson detects the path of the target shell via `find_program("sh")`,
which essentially does a lookup via `PATH`. This may easily lead to a
subtly-broken Git distribution when the build host has its shell in a
location that the target host doesn't know about.

Fix the issue by appending "/bin" to the custom program path, which
causes us to prefer "/bin/sh" over a `PATH`-based lookup. While
"/bin/sh" isn't standardized, this path tends to work alright on Linux
and BSD distributions. Furthermore, "/bin/sh" is also the path we pick
in our Makefile by default, which further demonstrates that this shell
fulfills our needs.

Note that we intentionally append, not prepend, to the custom program
path. This is because the program path can be configured by the user via
the `-Dsane_tool_path=` build option, which should take precedence over
any defaults we pick for the user.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>