Although Bimplerekkie's answer is good and reasonable, another explanation is possible, which was also hinted in a now deleted answer from DrFriedParts and accompanying comments by Ignacio Vasquez-Abrams.
Since I found relevant documentation about this other explanation, I think it is better to give it here (the true reason why that diode was put in the circuit must be asked to the designer of the Arduino board; either explanation is reasonable and it is possible that both were the reason which prompted that design decision).
That diode might be there to protect the reset pin from ESD (electro-static discharge) events. In fact most pins of the MCU have internal clamping diodes toward both the positive rail and ground, except the reset pin, which misses the diode toward the positive rail. This is shown in this application note from Microchip (excerpts):
4.9.2. Reset Pin Protection
During parallel programming, a 12V signal is connected to the Reset
pin. It is therefore no internal protection diode from Reset to VCC;
there is only one from GND to Reset. See the figure below.
Figure 4-6. Reset Pin Input Protection
To achieve the same protection on Reset as on other I/O pins, an
external diode should be connected from Reset to VCC. A normal
small-signal diode will do. In addition, a pull-up resistor
(10kΩ typical) and a small filter capacitor (4.7nF) should be
connected as shown in the figure below.
Figure 4-7. Recommended Reset Pin Connection
All this, of course, is not needed if Reset is connected
directly to VCC , but then external reset and In-System
Programming (ISP) is disabled, too.
If high ESD protection
of Reset is not required, or is achieved by other
components, the diode may be omitted. The resistor and
capacitor are still recommended for optimum Reset behavior.
This explanation is also backed by another document, i.e. Microchip's AN2519 - AVR® Microcontroller Hardware Design Considerations (excerpts):
3. Connection of RESET Pin on AVR Devices
ESD protection diode is not provided internally from Reset to VCC in order to allow HVPP. If HVPP is not used, it is recommended to add an ESD protection diode externally from Reset to VCC . Alternatively, a Zener diode can be used to limit the Reset voltage relative to GND. A Zener diode is highly
recommended in noisy environments. The components should be located physically close to the RESET pin of the AVR device. A recommended circuit of a Reset line is shown in the following circuit diagram.
Figure 3-1. Recommended Reset Pin Connection
Note that the capacitor value (100nF) and the pull-up resistor value (10kΩ) are both compatible with the indications given in those application notes. Therefore the designer may have just followed those indications.
BTW, the fact that my explanation could be the real motive in the designer's mind is also backed by what we find in another part of the Arduino UNO schematic (emphasis mine):
As you can see, there is a diode in the same position on the reset pin of the ATmega 16U2 MCU (the MCU that acts as a USB to UART bridge), too. In this case, though, there is no capacitor to discharge hence the ESD-protection explanation turns out to be quite reasonable.