scan.coverity: improve the modeling file of git.git

[Stefan Beller <stefanbeller@gmail.com>]

Hi Sam, John and Jeff,

I'm writing to you, as you're listed as the 
administrator of the git.git project 
on scan.coverity.com

We're currently seeing lots of false positives
as the xmalloc/xrealloc function is handled not properly
by coverity. There are lots of errors "Allocation too small for type"

Quoting (starting linenumbers are code) from some xrealloc ocurrence:


 95void *xrealloc(void *ptr, size_t size)
 96{
 97        void *ret;
 98
 99        memory_limit_check(size);
100        ret = realloc(ptr, size);

1. Condition "!ret", taking true branch   
2. Condition "!size", taking true branch

101        if (!ret && !size)
   
3. buffer_alloc: "realloc(void *, size_t)" which allocates 1 bytes based on "1UL".   
4. var_assign: Assigning: "ret" = storage allocated by "realloc(ptr, 1UL)".

102                ret = realloc(ptr, 1);
   
5. Condition "!ret", taking false branch

103        if (!ret) {
104                try_to_free_routine(size);
105                ret = realloc(ptr, size);
106                if (!ret && !size)
107                        ret = realloc(ptr, 1);
108                if (!ret)
109                        die("Out of memory, realloc failed");
110        }
   
6. return_dbuffer: Returning allocated array "ret".

111        return ret;
112}

However I have reviewed the function and I'd be pretty sure it would work as expected.
According to https://scan.coverity.com/tune we can upload a modelling file, 
which will allow us to supress such false positive errors.
I believe we'd need to put in the modelling file something like:

	// coverity[+alloc]
	void *xrealloc(void *ptr, size_t size);

and that should do. We'd not need to modify the git.git sources,
but just add such a declaration to the modelling file.

Does anyone of you administrators want to experiment with that?

Cheers,
Stefan