Documentation: add shell guidelines

Add the following guideline to Documentation/CodingGuidelines:

	Break overlong lines after "&&", "||", and "|", not before
	them; that way the command can continue to subsequent lines
	without backslash at the end.

And the following to t/README (since it is specific to writing tests):

	Pipes and $(git ...) should be avoided when they swallow exit
	codes of Git processes

Signed-off-by: Matthew DeVore <matvore@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/CodingGuidelines

@@ -118,6 +118,24 @@ For shell scripts specifically (not exhaustive):
 		do this
 	fi
 
+ - If a command sequence joined with && or || or | spans multiple
+   lines, put each command on a separate line and put && and || and |
+   operators at the end of each line, rather than the start. This
+   means you don't need to use \ to join lines, since the above
+   operators imply the sequence isn't finished.
+
+	(incorrect)
+	grep blob verify_pack_result \
+	| awk -f print_1.awk \
+	| sort >actual &&
+	...
+
+	(correct)
+	grep blob verify_pack_result |
+	awk -f print_1.awk |
+	sort >actual &&
+	...
+
  - We prefer "test" over "[ ... ]".
 
  - We do not write the noiseword "function" in front of shell

t/README

@@ -474,6 +474,33 @@ And here are the "don'ts:"
    platform commands; just use '! cmd'.  We are not in the business
    of verifying that the world given to us sanely works.
 
+ - Don't feed the output of a git command to a pipe, as in:
+
+     git -C repo ls-files |
+     xargs -n 1 basename |
+     grep foo
+
+   which will discard git's exit code and may mask a crash. In the
+   above example, all exit codes are ignored except grep's.
+
+   Instead, write the output of that command to a temporary
+   file with ">" or assign it to a variable with "x=$(git ...)" rather
+   than pipe it.
+
+ - Don't use command substitution in a way that discards git's exit
+   code. When assigning to a variable, the exit code is not discarded,
+   e.g.:
+
+     x=$(git cat-file -p $sha) &&
+     ...
+
+   is OK because a crash in "git cat-file" will cause the "&&" chain
+   to fail, but:
+
+     test "refs/heads/foo" = "$(git symbolic-ref HEAD)"
+
+   is not OK and a crash in git could go undetected.
+
  - Don't use perl without spelling it as "$PERL_PATH". This is to help
    our friends on Windows where the platform Perl often adds CR before
    the end of line, and they bundle Git with a version of Perl that