How tartaric acid gives Tollen's test?
Maxted[1] reported in 1926 that the reaction of tartaric acid with silver produces oxalic acid, formic acid, CO2 and water as products and that the stoichiometry is 6 moles of silver reduced per tartartic acid. He proposed the following possible net reactions:

$\ce{C4H6O6 + 6 Ag+ + 2H2O -&gt; 2(C2H2O4) + 6 Ag + 6 H+}$

$\ce{C4H6O6 + 6 Ag+ + 2H2O -&gt; C2H2O4 + CO2 + HCO2H + 6 Ag + 6 H+}$

$\ce{C4H6O6 + 6 Ag+ + 2H2O -&gt; 2(HCO2H) + 2CO2 + 6 Ag + 6 H+}$

For the mechanism, he concluded that initial decarboxylation to 2-hydroxy-3-oxo-propionate or splitting into two glyoxylic acid molecules were both possibilities. The glyoxylate could be oxidized to oxalic acid or oxidatively decarboxylated to formic acid, while the 2-hydroxy-3-oxo-propionate could split into glyoxylic acid and formic acid. 

I'm sure someone must have done more recent mechanistic work on this to pin it down further, but that gives a pretty good idea of the possibilities.  

[1] Maxted, D.R. (1926) CCLXXXVIII-The Oxidation of Tartartic Acid by Solutions of Silver Salts; J. Chem Soc. 129:2178.