Add Documentation/CodingGuidelines

Even if our code is quite a good documentation for our coding style,
some people seem to prefer a document describing it.

The part about the shell scripts is clearly just copied from one of
Junio's helpful mails, and some parts were added from comments by
Junio, Andreas Ericsson and Robin Rosenberg.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/Documentation/CodingGuidelines b/Documentation/CodingGuidelines
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+Like other projects, we also have some guidelines to keep to the
+code.  For git in general, three rough rules are:
+
+ - Most importantly, we never say "It's in POSIX; we'll happily
+   ignore your needs should your system not conform to it."
+   We live in the real world.
+
+ - However, we often say "Let's stay away from that construct,
+   it's not even in POSIX".
+
+ - In spite of the above two rules, we sometimes say "Although
+   this is not in POSIX, it (is so convenient | makes the code
+   much more readable | has other good characteristics) and
+   practically all the platforms we care about support it, so
+   let's use it".
+
+   Again, we live in the real world, and it is sometimes a
+   judgement call, the decision based more on real world
+   constraints people face than what the paper standard says.
+
+
+As for more concrete guidelines, just imitate the existing code
+(this is a good guideline, no matter which project you are
+contributing to).  But if you must have a list of rules,
+here they are.
+
+For shell scripts specifically (not exhaustive):
+
+ - We prefer $( ... ) for command substitution; unlike ``, it
+   properly nests.  It should have been the way Bourne spelled
+   it from day one, but unfortunately isn't.
+
+ - We use ${parameter-word} and its [-=?+] siblings, and their
+   colon'ed "unset or null" form.
+
+ - We use ${parameter#word} and its [#%] siblings, and their
+   doubled "longest matching" form.
+
+ - We use Arithmetic Expansion $(( ... )).
+
+ - No "Substring Expansion" ${parameter:offset:length}.
+
+ - No shell arrays.
+
+ - No strlen ${#parameter}.
+
+ - No regexp ${parameter/pattern/string}.
+
+ - We do not use Process Substitution <(list) or >(list).
+
+ - We prefer "test" over "[ ... ]".
+
+ - We do not write the noiseword "function" in front of shell
+   functions.
+
+For C programs:
+
+ - We use tabs to indent, and interpret tabs as taking up to
+   8 spaces.
+
+ - We try to keep to at most 80 characters per line.
+
+ - When declaring pointers, the star sides with the variable
+   name, i.e. "char *string", not "char* string" or
+   "char * string".  This makes it easier to understand code
+   like "char *string, c;".
+
+ - We avoid using braces unnecessarily.  I.e.
+
+       if (bla) {
+               x = 1;
+       }
+
+   is frowned upon.  A gray area is when the statement extends
+   over a few lines, and/or you have a lengthy comment atop of
+   it.  Also, like in the Linux kernel, if there is a long list
+   of "else if" statements, it can make sense to add braces to
+   single line blocks.
+
+ - Try to make your code understandable.  You may put comments
+   in, but comments invariably tend to stale out when the code
+   they were describing changes.  Often splitting a function
+   into two makes the intention of the code much clearer.
+
+ - Double negation is often harder to understand than no negation
+   at all.
+
+ - Some clever tricks, like using the !! operator with arithmetic
+   constructs, can be extremely confusing to others.  Avoid them,
+   unless there is a compelling reason to use them.
+
+ - Use the API.  No, really.  We have a strbuf (variable length
+   string), several arrays with the ALLOC_GROW() macro, a
+   path_list for sorted string lists, a hash map (mapping struct
+   objects) named "struct decorate", amongst other things.
+
+ - When you come up with an API, document it.
+
+ - The first #include in C files, except in platform specific
+   compat/ implementations, should be git-compat-util.h or another
+   header file that includes it, such as cache.h or builtin.h.
+
+ - If you are planning a new command, consider writing it in shell
+   or perl first, so that changes in semantics can be easily
+   changed and discussed.  Many git commands started out like
+   that, and a few are still scripts.
+
+ - Avoid introducing a new dependency into git. This means you
+   usually should stay away from scripting languages not already
+   used in the git core command set (unless your command is clearly
+   separate from it, such as an importer to convert random-scm-X
+   repositories to git).